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COMPARATIVE ELECTROCHEMISTRY, ELECTRONIC ABSORPTION
SPECTROSCOPY AND SPECTROELECTROCHEMISTRY OF THE
MONOMETALLIC RUTHENIUM POLYPYRIDYL COMPLEXES
[Ru(bpy)(dpb)2](PF6)2
[Ru(bpy)2(dpb)](PF6)2
[Ru(bpy)2(dpq)](PF6)2
[Ru(bpy)(dpq)2](PF6)2
by
Alan Duchovnay
Thesis submitted to the Faculty of the
Virginia Polytechnic Institute and State University
in partial fulfillment of the requirements for the degree of
MASTER OF SCIENCE
in
Chemistry
Karen J. Brewer, Chair
Joseph S. Merola
Brian E. Hanson
13 April 2011
Blacksburg, Virginia
Keywords: Spectroelectro Chemistry, Ruthenium
Copyright 2011, Alan Duchovnay
COMPARATIVE ELECTROCHEMISTRY, ELECTRONIC ABSORPTION
SPECTROSCOPY AND SPECTROELECTROCHEMISTRY OF THE
MONOMETALLIC RUTHENIUM POLYPYRIDYL COMPLEXES
[Ru(bpy)(dpb)2](PF6)2
[Ru(bpy)2(dpb)](PF6)2
[Ru(bpy)2(dpq)](PF6)2
[Ru(bpy)(dpq)2](PF6)2
Alan Duchovnay
ABSTRACT
The novel compound [Ru(bpy)(dpb)2(PF6)2 was synthesized, in a manner similar
to the literature synthesis of [Ru(bpy)(dpq)2(PF6)2. For the sake of completeness, the
related analogs, [Ru(bpy)2(dpb)](PF6)2, [Ru(bpy)2(dpq)](PF6)2 and [Ru(bpy)(dpq)2](PF6)2
were also synthesized. Alumina adsorption chromatography was used for purification
purposes. Liquid secondary ion mass spectroscopy was used to confirm identity of
compounds. The new compound contained 1% electroactive impurity as determined by
OSWV. Spectroelectrochemical studies were conducted with both a bulk H-cell and a
~0.2 mm pathlength, optically transparent thin layer electrode (OTTLE) cell. High
reversibility ( ca 99%) is possible with dilute solutions (ca 10-4 M ) and the OTTLE cell
as compared to ca 50% with the H-cell. Spectroelectrochemical data supported the
following electronic transitions for the new compound [Ru(bpy)(dpb)2](PF6)2 : (1) the Ru
(d) dpb MLCT at 552 nm, (2) a d d at 242 nm, a bpy   * at 285 nm. (3) The
location of the Ru (d) bpy MLCT peak is obscured by shoulders from 390-420 nm. (4)
The strong peak at 316 nm may be dpb   2*, the location of the lower energy
intraligand dpb   1* is uncertain. Upon oxidation of the metal center, no LMCT was
observed within the UV-VIS range. This is in direct contrast to the results of Gordon et
al. This author hypothesizes that their LMCT found in the visible region was actually the
result of incomplete electrochemical conversion and that a LMCT should be seen in the
NIR. The spectroelectrochemical properties of [Ru(bpy)(dpq)2](PF6)2 were also presented
for the first time. These results indicated that the 256 nm transition was d  d and not
bpy   2* as suggested by Rillema et al.
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DEDICATION
To my many mentors and my faithful dog, Blacker.
iv
ACKNOWLEDGMENTS
I would like to express my gratitude to my advisor, Dr. Karen Brewer. I would
also like to thank Kim Harrick and Ann Campbell for their help in obtaining mass
spectroscopy data.
v
TABLE OF CONTENTS
ABSTRACT.......................................................................................................................... ii
DEDICATION .................................................................................................................... iv
ACKNOWLEDGMENTS ......................................................................................................v
LIST OF ABBREVIATIONS ............................................................................................... xi
LIST OF TABLES.............................................................................................................. xii
LIST OF FIGURES ........................................................................................................... xiv
CHAPTER ONE: INTRODUCTION................................................................................... 1
Statement of Purpose .................................................................................................. 19
References .................................................................................................................... 20
CHAPTER TWO: EXPERIMENTAL METHODS ............................................................. 23
Materials ...................................................................................................................... 23
Electronic Absorption Spectroscopy ......................................................................... 23
Cyclic Voltammetry and Osteryoung Square Wave Voltammetry ........................ 23
Spectroelectrochemistry, Bulk (H-cell) Construction and Operation.................... 24
Optically transparent thin layer cell ......................................................................... 25
Mass Spectroscopy ...................................................................................................... 27
Synthesis ...................................................................................................................... 27
TBAH ........................................................................................................................ 27
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2,3-bis(2-pyridyl)quinoxaline (dpq) .......................................................................... 28
2,3-bis(2-pyridyl)benzoquinoxaline (dpb) ................................................................ 29
Bis(2,2’-bipyridine) ruthenium(II) dichloride. [Ru(bpy)2Cl2] .................................. 29
2,2’-bipyridine ruthenium(II) tetrachloride.[Ru(bpy)Cl4] ......................................... 29
Bis(2,2’-bipyridine)2,3-bis(2’-pyridyl)benzoquinoxaline)ruthenium(II)
Hexafluorophosphate. [Ru(bpy)2(dpb)](PF6)2 .......................................................... 30
(2,2’-bipyridine)(2,3-bis(2-pyridyl)quinoxaline)ruthenium(II) Hexafluorophosphate.
[Ru(bpy)(dpq)2](PF6)2: .............................................................................................. 30
(2,2’-bipyridine)(2,3-bis(2-pyridyl)benzoquinoxaline)ruthenium(II)
Hexafluorophosphate. [Ru(bpy)(dpb)2](PF6)2 ......................................................... 30
References .................................................................................................................... 32
CHAPTER THREE: RESULTS ......................................................................................... 33
Identification using Liquid Secondary Ion Mass Spectroscopy ............................. 33
Electrochemistry of Background Electrolyte and Ligands ..................................... 38
Electrochemistry-OSWV ............................................................................................ 42
Electrochemistry-Cyclic Voltammetry ...................................................................... 46
UV-Vis Spectroscopy .................................................................................................. 50
Monometallic Ruthenium Complexes ...................................................................... 51
Spectroelectrochemistry ............................................................................................ 51
[Ru(bpy)2(dpq)](PF6)2 ............................................................................................... 52
[Ru(bpy)(dpq)2](PF6)2 ............................................................................................... 58
vii
[Ru(bpy)(dpb)2](PF6)2 ............................................................................................... 61
References .................................................................................................................... 64
CHAPTER FOUR: DISCUSSION .................................................................................... 65
Synthesis and Purification.......................................................................................... 65
Identification of Products ........................................................................................... 66
Determination of Purity.............................................................................................. 66
Cyclic Voltammetry .................................................................................................... 67
Electronic Absorption Spectroscopy and Spectroelectrochemistry ....................... 70
UV-Vis Spectra of [Ru(bpy)2(dpq)](PF6)2 ................................................................. 75
Spectroelectrochemistry of [Ru(bpy)2(dpq)](PF6)2 .................................................. 76
Impact of Oxidation at 1.90 V .................................................................................. 76
Interpretation ............................................................................................................. 77
Impact of First Reduction at –1.00 V ........................................................................ 78
Interpretation ............................................................................................................. 78
Impact of Second Reduction at –1.55 V ................................................................... 79
The UV-Vis Spectrum of [Ru(bpy)(dpq)2](PF6)2 ..................................................... 80
Spectroelectrochemistry of [Ru(bpy)(dpq)2](PF6)2 .................................................. 81
Impact of Oxidation at 1.80 V .................................................................................. 82
Interpretation ............................................................................................................. 82
Impact of the First Reduction at –0.70 V .................................................................. 83
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Interpretation ............................................................................................................. 83
Impact of the Second Reduction at –1.00 V ............................................................. 84
Interpretation ............................................................................................................. 84
Comparison between [Ru(bpy)2(dpq)](PF6)2 and [Ru(bpy)(dpq)2](PF6)2: ............ 85
UV-Vis Spectrum of [Ru(bpy)2(dpb])(PF6)2............................................................. 86
UV-Vis Spectra of [Ru(bpy)(dpb)2](PF6)2 ................................................................ 88
Spectroelectrochemistry of [Ru(bpy)(dpb)2](PF6)2: ................................................. 88
Impact of Oxidation at 1.75 V .................................................................................. 88
Interpretation ............................................................................................................. 88
Impact of the First Reduction at –0.50 V .................................................................. 89
Interpretation ............................................................................................................. 89
Impact of the Second Reduction at –1.0 V ............................................................... 90
Interpretation ............................................................................................................. 90
Comparison between [Ru(bpy)2(dpb)](PF6)2 and [Ru(bpy)(dpb)2](PF6)2 ............. 91
Comparison between [Ru(bpy)(dpq)2](PF6)2 and Ru(bpy)(dpb)2](PF6)2............ 92
References .................................................................................................................... 93
CHAPTER FIVE: CONCLUSIONS AND FUTURE WORK............................................. 94
Synthesis ...................................................................................................................... 94
Electrochemistry ......................................................................................................... 95
Spectroelectrochemical Experimental Design .......................................................... 96
ix
Summary of Spectroelectrochemistry Conclusions ................................................. 96
Future Work ................................................................................................................ 98
References .................................................................................................................. 100
APPENDIX FIGURE 1.1. Diasteriomers of [Ru(bpy)(dpb)2]+2.................................... 101
APPENDIX FIGURE 3.1. LSIMS+ of the matrix glycerol. MW 92+1. .......................... 102
APPENDIX FIGURE 3.2. LSIMS+ of the matrix nitro-benzyl alcohol. MW 153+1 ..... 103
APPENDIX FIGURE 3.3. LSIMS+ of free ligand bpy in glycerol. MW 156+1. ............ 104
APPENDIX FIGURE 3.4. LSIMS+ of free ligand dpq in glycerol. MW 284+1. ............ 105
APPENDIX FIGURE 3.5. LSIMS+ of free ligand dpb in glycerol. MW 334+1. ............ 106
APPENDIX FIGURE 3.6. Electrospray (+) MS of Ru(bpy)Cl4 in acetonitrile/water. MW=
399................................................................................................................................... 107
APPENDIX FIGURE 3.7. UV-Vis spectroelectrochemistry of ferrocene. ( __= 0 V, ….. =
1.7 V). ............................................................................................................................. 108
AAPPENDIX FIGURE 3.8. LSIMS- of RuCl33H2O in NBA. MW= 255+1................. 109
APPENDIX FIGURE 3.9. The electronic absorption spectra of the two fractions of
Ru(bpy)Cl4 in water. ( Where bpy= 2,2’ bipyridine). ..................................................... 110
x
LIST OF ABBREVIATIONS
Bpy
2,2’-Bipyridine
BL
Bridging Ligand
CT
Charge Transfer
dpb
2,3-Bis(2-pyridyl)benzoquinaxoline
dpp
2,3-Bis(2-pyridyl)pyrazine
dpq
2,3-Bis(2-pyridyl)quinoxaline
ES
Excited State
GS
Ground State
HOMO
Highest-Occupied Molecular Orbital
IL
Intraligand
LF
Ligand Field
LSIMS
Liquid Secondary Ion Mass Spectroscopy
LMCT
Ligand to Metal Charge Transfer
LUMO
Lowest Unoccupied Molecular Orbital
MLCT
Metal to Ligand Charge Transfer
MO
Molecular Orbital
NHE
Normal Hydrogen Electrode
TBAH
Tetrabutylammonium hexafluorophospate
UV
Ultraviolet
Vis
Visible
xi
LIST OF TABLES
Table 1.1. Cyclic voltammetric data for [Ru(bpy)3](BF4)2 (where bpy = 2,2’bipyridine).14 ....................................................................................................................... 5
Table 1.2. A comparison of cyclic voltammetric Data for [Ru(bpy)2(dpb)](PF6)2 (where
bpy = 2,2’- bipyridine and dpb = 2,3-bis(2-pyridyl)benzo-quinaxoline).a ........................ 15
Table 1.3. Summary of Electrochemical Data: Cyclic voltammetric data for
[Ru(bpy)2(dpq)](PF6)2, [Ru(bpy)2(dpb)](PF6)2, [Ru(bpy)(dpq)2](PF6)2 and
[Ru(bpy)(dpb)2](PF6)2. ...................................................................................................... 17
Table 1.4 Summary of Electronic Absorption Spectroscopy: The electronic absorption
spectroscopy of [Ru(bpy)2(dpq)](PF6)2, [Ru(bpy)2(dpb)](PF6)2, [Ru(bpy)(dpq)2](PF6)2 and
[Ru(bpy)(dpb)2](PF6)2 in acetonitrile. ............................................................................... 18
Table 3.1. Cyclic voltammetric data for a series of ruthenium (II) complexes
incorporating polypyridyl ligands ( where bpy = 2,2’-bipyridine, dpq = 2,3-bis(2pyridyl)quinoxaline , dpb =2,3-bis(2-pyridyl)benzoquinoxaline).a .................................. 47
Table 4.1. Reduction potentials vs NHE in acetonitrile with 0.1 M TBAH at room
temperature for a series of polypyridyl ligands and ruthenium complexes....................... 69
Table 4.2. Electronic transitions of polyazine complexes of ruthenium and the impact of
metal oxidation or ligand reduction upon these transitions. ............................................. 73
Table 4.3. UV-Vis electronic absorption spectra of [Ru(bpy)2(dpq)](PF6)2 , in acetonitrile,
at T = 20+/- oC, according to Rillema and Mack.2b .......................................................... 75
Table 4.4. UV-Vis absorption spectra of [Ru(bpy)2(dpq)](PF6)2 , in acetonitrile, at T= 25
o
C, according to Gordon et al.10 ........................................................................................ 76
Table 4.5. Summary for The UV-Vis electronic absorption spectra of
[Ru(bpy)2(dpq)](PF6)2 in acetonitrile at room temperature. .............................................. 80
Table 4.6. UV-Vis electronic absorption spectra of [Ru(bpy)(dpq)2](PF6)2 , in acetonitrile,
at T= 20+/-1 oC, According to Rillema et al.2c ................................................................. 81
Table 4.7. Summary of UV-Vis electronic absorption spectra of [Ru(bpy)(dpq)2](PF6)2 in
acetonitrile......................................................................................................................... 85
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Table 4.8. UV-Vis electronic absorption spectra of [Ru(bpy)2(dpb)](PF6)2 in acetonitrile
at 25oC by Gordon et al.10 ................................................................................................. 87
Table 4.9. Summary of UV-Vis electronic absorption spectra of [Ru(bpy)(dpb)2](PF6)2, in
acetonitrile, at room temperature. ..................................................................................... 91
Table 4.10. Summary of UV-Vis electronic absorption spectra of [Ru(bpy)2(dpb)](PF6)2
in acetonitrile. ................................................................................................................... 92
xiii
LIST OF FIGURES
Figure 1.1 Electronic absorption spectra of [Ru(bpy)3](PF6)2 in acetonitrile (where bpy =
2,2’ bipyridine).................................................................................................................... 3
Figure 1.2. Absorption spectra of [Ru(bpy)3]z complexes: ................................................. 4
Figure 1.3. Polypyridyl ligands used in this study. ........................................................... 12
Figure 2.1. H-cell for bulk spectroelectrochemistry. ........................................................ 25
Figure 2.2. The OTTLE cell of Krejcik et al.5 .................................................................. 26
Figure 2.3. Background CV of 0.1 M TBAH in acetonitrile at room temperature using a
Ag/AgCl reference electrode............................................................................................. 28
Figure 3.1. LSIMS of [Ru(bpy)2(dpb)](PF6)2 ................................................................... 35
Figure 3.2. LSIMS of [Ru(bpy)2(dpq)](PF6)2. .................................................................. 36
Figure 3.3. LSIMS of [Ru(bpy)2(dpq)](PF6)2. .................................................................. 37
Figure 3.4. LSIMS of [Ru(bpy)(dpb)2](PF6)2 . .................................................................. 38
Figure 3.5. OSWV of background electrolyte, 0 to –1.8 V. 0.1 M TBAH in acetonitrile. 39
Figure 3.6. OSWV of background electrolyte, 0 to 1.8 V. 0.1 M TBAH, in acetonitrile. 40
Figure 3.7. CV of ferrocene oxidation with IR (internal resistance) compensation and
platinum electrode in acetonitrile. Ep = 56 mV. ............................................................ 40
Figure 3.8. CV of a high concentration of ferrocene with no IR compensation in
acetonitrile. Ep = 462 mV .............................................................................................. 41
Figure 3.9. Osteryoung square wave voltammogram of [Ru(bpy)2(dpb)](PF6)2 (where
bpy= 2,2’ bipyridine, dpb= 2,3-bis(2-pyridyl)benzoquinoxaline and using a Ag/AgCl
reference electrode). .......................................................................................................... 42
xiv
Figure 3.10. Osteryoung square wave voltammogram of [Ru(bpy)2(dpq)](PF6)2 (where
bpy= 2,2’ bipyridine, dpq= 2,3-bis(2-pyridyl)quinoxaline and using a Ag/AgCl reference
electrode)........................................................................................................................... 43
Figure 3.11. Osteryoung square wave voltammogram of [Ru(bpy)(dpq)2](PF6)2 (where
bpy= 2,2’ bipyridine, dpq= 2,3-bis(2-pyridyl)quinoxaline and using a Ag/AgCl reference
electrode)........................................................................................................................... 44
Figure 3.12. Osteryoung square wave voltammogram of [Ru(bpy)(dpb)2](PF6)2 (where
bpy= 2,2’ bipyridine, dpb = 2,3-bis(2-pyridyl)benzoquinoxaline and using a Ag/AgCl
reference electrode). .......................................................................................................... 45
Figure 3.13. Osteryoung square wave voltammogram of [Ru(bpy)3](PF6)2 (where bpy =
2,2’ bipyridine and using a Ag/AgCl reference electrode). .............................................. 46
Figure 3.14. Cyclic voltammogram of [Ru(bpy)2(dpq)](PF6)2 (where bpy= 2,2’bipyridine, dpq = 2,3-bis(2-pyridyl)quinoxaline and using a Ag/AgCl reference electrode
with a scan rate of 200 mV/sec). ....................................................................................... 48
Figure 3.15. Cyclic voltammogram of [Ru(bpy)2(dpb)](PF6)2 (where bpy= 2,2’bipyridine, dpb = 2,3-bis(2-pyridyl)benzoquinoxaline and using a Ag/AgCl reference
electrode with a scan rate of 200 mV/sec). ....................................................................... 48
Figure 3.16. Cyclic voltammogram of [Ru(bpy)(dpq)2](PF6)2 (where bpy= 2,2’bipyridine, dpq = 2,3-bis(2-pyridyl)quinoxaline and using a Ag/AgCl reference electrode
with a scan rate of 200 mV/sec). ....................................................................................... 49
Figure 3.17. Cyclic voltammogram of [Ru(bpy)(dpb)2](PF6)2 (where bpy = 2,2’bipyridine, dpb = 2,3-bis(2-pyridyl)benzoquinoxaline and using a Ag/AgCl reference
electrode with a scan rate of 200 mV/sec). ....................................................................... 49
Figure 3.18. The comparative UV-Vis spectra (molar absorptivity vs wavelength) of the
ligands bpy (….), (where bpy = 2,2’-bipyridine) dpq (___), (where dpq =2,3-bis(2pyridyl)quinoxaline) and dpb (_ _ _), (where dpb =2,3-bis(2-pyridyl)benzoquinoxaline).
........................................................................................................................................... 50
Figure 3.19. The comparative electronic absorption spectra of [Ru(bpy)2dpq](PF6)2 (light
blue), [Ru(bpy)2(dpb)](PF6)2 (dark blue), [Ru(bpy)(dpq)2](PF6)2 (black) and
[Ru(bpy)(dpb)2](PF6)2 (red) in acetonitrile at room temperature. .................................... 51
xv
Figure 3.20. Oxidative spectroelectrochemistry of [Ru(bpy)2(dpq)](PF6)2 with H-cell.
(___= 0V, _ _ _ = 1.89V, ….. = -0.25V). (Where bpy = 2,2’-bipyridine, dpq = 2,3-bis(2pyridyl)quinoxaline). 0.1M TBAH in CH3CN at room temperature. ............................... 53
Figure 3.21. Oxidative spectroelectrochemistry of [Ru(bpy)2(dpq)](PF6)2 with OTTLE
cell. (Black = 0V, red = 1.90V, blue = 0V). (Where bpy = 2,2’-bipyridine, dpq = 2,3bis(2-pyridyl)quinoxaline). 0.1M TBAH in CH3CN at room temperature. ...................... 54
Figure 3.22. First reduction of [Ru(bpy)2(dpq)](PF6)2 with H-cell. (___ = 0V, _ _ _=
S-1.00V, …. = 0.25V). (Where bpy= 2,2’-bipyridine, dpq = 2,3-bis(2pyridyl)quinoxaline). 0.1M TBAH in CH3CN at room temperature. .............................. 55
Figure 3.23. First reduction of [Ru(bpy)2(dpq)](PF6)2 with OTTLE cell. (Black = 0V, red
= -1.00V, blue = 0V). (Where bpy = 2,2’-bipyridine, dpq = 2,3-bis(2pyridyl)quinoxaline). 0.1 M TBAH in CH3CN at room temperature. ............................. 56
Figure 3.24. First, second and third reduction of [Ru(bpy)2(dpq)](PF6)2 with the OTTLE
cell. (Where bpy = 2,2’-bipyridine, dpq = 2,3-bis(2-pyridyl)quinoxaline). (Black = 1.00V, red = -1.55V, blue = 1.90V). 0.1 M TBAH in CH3CN at room temperature. ...... 57
Figure 3.25. Oxidative spectroelectrochemistry of [Ru(bpy)(dpq)2](PF6)2 with OTTLE
cell. (Where bpy= 2,2’-bipyridine, dpq = 2,3-bis(2-pyridyl)quinoxaline). (Black = 0 V,
red = 1.80 V, blue = 0 V). 0.1 M TBAH in CH3CN at room temperature ....................... 58
Figure 3.26. First reductive spectroelectrochemistry of [Ru(bpy)(dpq)2](PF6)2 with
OTTLE cell. (Where bpy = 2,2’-bipyridine, dpq = 2,3-bis(2-pyridyl)quinoxaline). (Black
= 0 V, red = -0.70 V, blue = 0V). 0.1 M TBAH in CH3CN at room temperature. ........... 59
Figure 3.27. First and second reductive spectroelectrochemistry of
[Ru(bpy)(dpq)2](PF6)2 with OTTLE cell. (Where bpy = 2,2’-bipyridine, dpq = 2,3-bis(2pyridyl)quinoxaline). Black = 0 V, red = -0.70 V, blue = -1.00 V. 0.1 M TBAH in
CH3CN at room temperature. ........................................................................................... 60
Figure 3.28. The oxidative spectroelectrochemistry of [Ru(bpy)(dpb)2](PF6)2 with the
OTTLE cell. (Black = 0 V, red = 1.75 V, blue = 0 V). 0.1M TBAH in CH3CN at room
temperature. ...................................................................................................................... 61
Figure 3.29. The first reductive spectroelectrochemistry of [Ru(bpy)(dpb)2](PF6)2 with the
OTTLE cell. ( Solid line = 0 V, dash and dots = -0.50 V, dots = 0 V). 0.1 M TBAH in
CH3CN at room temperature. ............................................................................................ 62
xvi
Figure 3.30. Composite of parent complex, first, second and third reduction
spectroelectrochemistry of [Ru(bpy)(dpb)2] (PF6)2 with the OTTLE cell. ....................... 63
xvii
CHAPTER ONE: INTRODUCTION
The aim of this research was to probe the redox and spectroscopic properties of
ruthenium(II) complexes of the polyazine bridging ligand 2,3-bis(2-pyridyl)benzoquinoxaline
and the structurally related 2,3-bis(2-pyridyl)quinoxaline analogs.
A brief overview of the history and chemistry of ruthenium is relevant to
understanding the chemistry of ruthenium polypyridyl compounds. Ruthenium was first found
in 1826, by G. W. Osann, from platinum residues found in Ruthenia, a district of Russia. It
was isolated and characterized by K. K. Klaus in 1844. It occurs naturally in the mineral
laurite (Ru,Os)S2 and in the alloy osmiridium (Os, Ru and Ir).1 Being inert to acids including
aqua regia, the alloy is dissolved in an alkaline oxidizing mixture of KOH and KNO3, to form
the salt K2RuO4. The osmate cation is then oxidized to OsO4 by acidification and removed by
distillation. The remaining ruthenium complex is converted to RuO4 by NaOH addition and
isolated by distillation.2 The addition of concentrated HCl to the RuO4(aq) yields the watersoluble compound RuCl3 3H2O.3 The actual composition includes both the monomeric
Ru(III) form and polynuclear ruthenium (IV) oxy and hydroxy chloro species .4 Metallic
ruthenium is produced from RuO4 by reduction with H2.5
Ruthenium exhibits a large range of oxidation states, from 0 (Ru metallic and Ru(CO)5
to +8 (RuO4) and subsequently a large number of ruthenium complexes have been
synthesized. Ruthenium’s coordination chemistry as a second row, Group VIII, transition
metal is greatly influenced by the stability of the low spin d6 configuration. Ru(II) forms stable
octahedral complexes that are substitutionally inert with many -acid ligands such as the
chelating ligand 2,2’-bipyridine (bpy).
1
Intense interest in the ruthenium polypyridyl complexes started with the discovery of
the dramatic luminescence of [Ru(bpy)3]Cl2 in 1959.6 It is therefore appropriate to review the
Ultraviolet-Visible spectroscopy, electrochemistry, and spectroelectrochemistry of this
compound.
The Ultraviolet-Visible electronic absorption spectrum of [Ru(bpy)3](PF6)2 in
acetonitrile is shown in Figure 1.1. The lowest energy peak at 450 nm is a Ru(II) (d)  bpy
1
(*) charge transfer ( bpy MLCT). At low temperature (77K), in an ethanol- methanol glass
there is a shoulder at 550 nm which is thought to be a 3MLCT.7 The molar absorptivity for this
spin forbidden transition is 600 cm
-1
M
-1
1
(vs 1.45 x 104 cm-1 M-1 for the MLCT at 452
*
nm).7 It is also possible to have 1MLCT’s terminating on higher energy  orbital. Balzani et
al.8 assign a higher energy MLCT at 240 nm whereas Kalyanasundaram7 describes two higher
energy MLCTs at 238 nm and 250 nm. Intraligand transitions also occur. The peak at 288 nm
*
is assigned to a bpy   1 transition. Higher energy peaks at 185 nm and 205 nm are
*
*
attributed to bpy  3 and bpy  2 transitions.9
2
Figure 1.1 Electronic absorption spectra of [Ru(bpy)3](PF6)2 in acetonitrile (where bpy =
2,2’ bipyridine).
*
Although the metal centered d  d is a Laporte forbidden transition (gerade to
gerade), Balzani et al.,8 Lytle and Hercules 10 and Harriman 11 assign the small shoulders at 23
nm, 345 nm and 250 nm and peak at 238 nm to metal centered transitions. Note that
Kalyanasundaram ascribes these absorbances to higher energy MLCTs.7
The determination of the assignments of electronic absorption processes is aided by the
use of spectroelectrochemistry.12,13 It is surprising that to date the only published UV-Vis, near
IR spectroelectrochemical study is from 1981.14 [Ru(bpy)3]z is stable in a variety of oxidation
states with z = 2 (neutral with (BF4)2 counterion, 1 (one electron reduction), 0 (two electron
3
reduction) and -1 (three electron reduction). These results are shown in Figure 1.2 and Table
1.1. ( The appropriate reduction potentials were determined by cyclic voltammetry.)
Figure 1.2. Absorption spectra of [Ru(bpy)3]z complexes: (a) z = 2+; (b) z = 1+; (c) z = 0;
(d) z = 1-; complexes dissolved in dimethyl sulphoxide (UV-Vis region,  = optical extinction
coefficient in mol-1 dm3 cm-1). Heath, G. A.; Yellowlees, L. J.; Braterman, P. S. J. C. S. Chem.
Comm. 1981, 287. Reproduced by permission of The Royal Society of Chemistry.
4
Table 1.1. Cyclic voltammetric data for [Ru(bpy)3](BF4)2 (where bpy = 2,2’-bipyridine).14
E1/2 (V vs NHE)
+1.51
Assignment
-1.07
Ru(II)/Ru(III)
bpy/bpy
-1.26
bpy/bpy-
-1.53
bpy/bpy-
The lowest energy transition for ruthenium(II) polypyridyl complexes is typically the Ru (d)
 bpy (*) , metal to ligand charge transfer. For [Ru(bpy)3] +2 , this occurs at ~450 nm (see
Figure 1.1 and 1.2). The location of the higher energy MLCTs cannot be determined by visual
inspection of the spectrum of the +2 oxidation state.
The spectroscopy of the +3 oxidation state, Ru(III) adds significant spectral data.
Upon oxidation from Ru(II) to Ru(III), the d orbitals are stabilized, if this were not the case,
then the subsequent oxidation ( +3 to +4) would be potentially observable. As a consequence
of the lowering of energy of the d orbitals, the MLCT is shifted out of the UV-Vis window.
Therefore, there should be a loss of the 450 nm and 238 nm peaks and the 250 nm shoulder.
(Kalyanasundaram proposes that a ligand to Ru(III) charge transfer (LMCT) will be found to
*
the red of the lost MLCT.)15 The metal- centered, d  d shoulders, at 323 nm and 345 nm
*
should also be shifted out of the UV range. The bpy    transitions, at 288 nm, 205 nm
and 185 nm should be slightly red shifted, due to the decreased backbonding, with the lowest
energy transition experiencing the greatest red shift.16
The spectra of the one electron reduced form [RuII(bpy-)(bpy)2]+1 is labeled b in Figure
*
1.2. The newly occupied bpy  MO shifts and no longer contributes to the MLCT at 455 nm.
5
*
The remaining bpy  MOs are destabilized due to increased backbonding. It would seem
logical that the new MLCT would be found at a higher energy. The authors assign the lower
energy peak at 474 nm as the new MLCT, implying that the metal center is destabilized due to
*
decreased backbonding. The bpy    transition has red shifted from 286 nm to 292 nm
and decreased in intensity, lending further support to the overall destabilization of the metal
center and accompanying bpy  MO. The new peak at 342 nm is assigned as a poorly defined
bpy internal * * transition. Upon the second reduction , the MLCT has again red shifted
from 474 nm to 481 nm. The peak at 342 nm has increased in intensity and red shifted 3 nm.
*
The bpy    has red shifted to 296 nm and decreased in intensity.
*
With the third reduction, each bpy  MO has undergone a one electron reduction.
This should result in the loss of the MLCT at 481 nm. The authors do not assign an MLCT for
this reduction but spectrum d shows a peak slightly red shifted from the previously assigned
MLCT. In addition to the above assignments, the same authors, in another paper, propose a
*
ligand-ligand inter-valence charge transfer (IVCT), between a reduced bpy  and a
destabilized, unoccupied bpy * MO, at 2220 nm.17
In summary, UV-Vis spectroelectrochemistry of a one electron reduced or oxidized
form of a complex can confirm the assignment of the lowest energy MLCTs and strongly
substantiate the second lowest MLCT. These are the most important transitions in
photosensitization applications. The verification of other transitions is less robust. Upon a one
electron reduction, the remaining neutral ligand * MOs are destablized due to increased
6
backbonding. This increases the HOMO-LUMO energy gap, thereby creating transitions not
usually present in ground state complexes. Thus the results of higher level reductions are very
much open to different interpretations and explanations and frequently ask more questions
than they answer.
As mentioned above, the lowest energy MLCT is the most significant transition in
photochemistry. One extraordinary characteristic of MLCTs is that the resulting excited state
molecule is both a better oxidant and reductant than its ground state precursor. Furthermore,
the ruthenium polypyridyl complexes are not usually subject to photo-dissociation or ligand
substitution in aqueous solutions at room temperature. 18
The following Latimer diagram19 will be used to illustrate this phenomena.
*
Ru(bpy)3 +2
3
MLCT
- 2.12eV
0. 84 V(E
red
)
Ru(bpy)3+1
red
-1.28 V(E )
0.86 V(E
Ru(bpy)3+2
ox
)
Ru(bpy)3+3
ox
-1.26 V(E )
The redox potential of the excited state, Ru(bpy)3+2 cannot be measured directly.
The oxidation potential, Ru(bpy)3+2*  e + Ru(bpy)3+3 can be approximated as the sum of
the E00, Ru(bpy)3+2  Ru(bpy)3+2* (2.12 eV) plus the ground state oxidation potential,
7
ox
Ru(bpy)3+2  Ru(bpy)3+3 + e (-1.26V, E ). Thus the excited state is a stronger reducing
ox
ox
agent (0.86V) (E ) than the ground state Ru(bpy)3+2 (-1.26V)(E ). The strength of the
excited state reduction potential, likewise, follows the same argument.
The first documented photosensitization reaction with Ru(bpy)3 +2 was by Adamson
and Demas 20 in 1971.
Ru(bpy)3+2
PtCl4-2 + H20No reaction
Ru(bpy)3+2
PtCl4-2 +H20PtCl3(H20) + Cl
hv
The Ru(bpy)3+2 acts as a photosensitizer 21 or light absorbing sensitizer 22 (LAS),
converting solar energy to available free chemical energy. Two possible mechanisms are
possible, energy transfer and outer-sphere electron transfer. The above reaction is one of the
few energy transfer quenching reactions, electron transfer reactions being by far the more
common.23
8
The best known example of LAS and bi- molecular electron transfer is photosynthesis.
It is therefore not surprising that one major thrust of ruthenium polypyridyl research has been
in the area of solar energy conversion. The practical and efficient conversion of water into
hydrogen and oxygen using Ru(bpy)3+2 as the LAS is a specific area of ongoing research.24
An efficient and functioning LAS for photoredox chemistry must fulfill several
criteria. (1a) The molecule’s absorption spectrum must contain the wavelengths of the
MLCTs. (1b) The molar absorptivity must be high. (2) The molecule cannot photodissociate,
i.e. there can not be loss of ligands or ligand substitution. (3) The reduction of the ligand and
the oxidation of the metal center must be electrochemically reversible. (4) The desired redox
reaction must be thermodynamically favorable. (5) The excited state lifetime must be
sufficiently long to permit the reaction to occur.25
The utility of Ru(bpy)3+2 as a LAS is limited by (1) single electron transfer, (2) a
narrow absorption range of the MLCT, and (3) the necessity for bi-molecular collisions.26
These limitations can be circumvented by the synthesis of multimetallic transition metal
systems with polypyridyl bridging ligands and bpy termininal ligands.27 The four compounds,
[Ru(bpy) (dpb)2](PF6)2, [Ru(bpy)2(dpb)](PF6)2 , [Ru(bpy)2(dpq)](PF6)2 ,
[Ru(bpy)(dpb)2](PF6)2, [Ru(bpy)(dpq)2](PF6)2, which are the subject of this thesis, are
examples of monometallic LAS, which utilize the bridging ligands 2,3-bis(2pyridyl)quinoxaline (dpq) and 2,3-bis(2- bipyridyl)benzoquinoxaline (dpb).
The ligand dpq (see Figure 1.3) was first synthesized by Goodwin and Lions in 1959.28
The mechanism is a Schiff base condensation between the bis-ketone 2,2’-pyridil and the
diamine o-phenylenediamine. The synthesis of dpb is a modification of the above synthesis in
9
which o- phenylenediamine is substituted by 2,3-diaminonapthalene. Together with the
bridging ligand dpp28, dpq , dpb29 and the terminal ligand bpy represent a homologous series
of diimine ligands.
The incorporation of these ligands within the octahedrally coordinating Ru(II) allows
*
for the tuning of the MLCTs , ligand-centered transitions (   ) and excited state lifetimes
(according to the energy gap law) 30 as follows. Polypyridyl ligands have nitrogens with lone
pairs which form dative bonds with the 6 empty atomic orbitals of ruthenium. The actual
nature of the metal ligand bonding is more complex. From a molecular orbital picture, the
ligands can donate electron density (-donation) and accept electron density (-backbonding)
from the metal center. The degree of stabilization or destabilization is a result of the balance
between these two opposing effects. The strength of the -donation is reflected in the pKa’s
of these ligands, for which bpy>dpp>dpq>dpb. The degree of -backbonding is a function of
*
31
the extent of conjugation and the presence of empty  MOs, with dpb>dpq>dpp>bpy.
When the metal center is destabilized by -donation or decreased backbonding, the HOMOLUMO gap is thereby decreased and the absorption wavelength of the MLCT is diminished.
The excited state lifetime is concomitantly decreased.
Background on [Ru(bpy)2(dpp)](ClO4)2 : This compound was first synthesized by
Gafney et al. In 1984 32 by reacting cis-[Ru(bpy)2Cl2] and dpp in 95% ethanol for 72 hours.
The perchlorate salt was precipitated by the addition of aqueous NaClO4 to the hot solution
and cooling. The precipitate was recrystallized several times in 1:1 ethanol –water. Elemental
analysis showed the compound to have two molecules of hydration. The UV-Vis absorption
10
*
spectra showed the typical bpy    transition (~286 nm) found for [Ru(bpy)3] +2. In the
visible region there was one low energy peak at 430 nm and one shoulder at 480 nm. The
resonance Raman spectrum confirmed that the shoulder was the Ru (d) dpp (d*) MLCT
and the 430 nm peak was the Ru (d) bpy (*) MLCT.
Berger 33 and Gordon and Smith 34 have published the electro-chemistry,
spectroelectrochemistry and UV-Vis absorption and emission spectra . From cyclic
voltammetry, it was determined that [Ru(bpy)2(dpp)]+2 undergoes one, one electron
reversible oxidation (in acetonitrile at +1.45V vs Ag/AgCl) and three reversible, one electron
reductions in DMF at –0.91V, -1.32V and –1.56V vs Ag/AgCl.
11
bpy= 2,2’-bipyridine
Figure 1.3. Polypyridyl ligands used in this study.
12
Gordon and Smith found that upon oxidation (+1.5 V) the dpp MLCT at 470 nm
was lost and a new lower energy peak at 705 nm grew in, for which they assigned a LMCT .
Surprisingly, they do not note that the bpy MLCT at 424 nm is still present after the oxidation,
( it should not be present). Furthermore, their spectra of the first reduction was generated at
–0.90 V but their recorded CV data finds the reduction to be at –1.06 V ( both values vs SCE).
Background on [Ru(bpy)2(dpq)](PF6)2 : This compound was first synthesized by Rillema and
Mack in 1982.35 Ru(bpy)2Cl22H2O was reacted with AgPF6 in ~50 ml acetone. After
removal of AgCl, dpq was added to the solution and the resulting slurry was refluxed under
nitrogen for 24 hours. The resulting solution was reduced in volume and added to an excess of
ether. The product was dried under vacuum and purified on an alumina with a 1:1 acetonitrilemethylene chloride mobile phase. Cyclic voltammetry was used to determine the oxidation
potential (1.41V vs SSCE) and first two reduction potentials ( -0.78 V and
–1.41 V vs SSCE). The following assignments were made for the UV-Vis absorption spectra:
3
Ru (d)  dpq (*) MLCT (515 nm, =8.1x10 , Ru (d)  bpy (*) MLCT (427 nm (sh)).
Rillema et al.37 have also assigned the shoulders at 391 nm and 350 nm to Ru (d)  dpq
(2*) and Ru (d)  bpy (2*). The authors did not attempt to distinguish between the dpq 
*
*
  and the bpy    transitions.
Gordon et al.36 studied the spectroelectrochemistry of the oxidized form of
[Ru(bpy)2(dpq)](PF6)2 generated at +1.4V vs SCE and the first reduced form generated at –
0.80V vs SCE. Upon oxidation the two lowest energy peaks, 515 nm and 425 nm (sh)
disappeared, verifying their assignments of the Ru (d)  dpq (*) MLCT and Ru (d)
13
 bpy (*) MLCT respectively. In accordance with his data of the oxidation of
[Ru(bpy)2(dpp)]+2 , Gordon observed a lower energy peak at ~550 nm, having generated
[Ru(bpy)2(dpq)]3+, which he assigned as an LMCT transition. Upon reduction of the dpq
ligand, the dpq based MLCT was bleached and the remaining bpy based MLCT red shifted
due to destabilization of the metal center due to decreased backbonding. He did not use his
data to distinguish between bpy vs dpq transitions.
Background on [Ru(bpy)(dpq)2](PF6)2 : Rillema et al.37 synthesized
[Ru(bpy)(dpq)2](PF6)2 by reacting two parts dpq and one part [Ru(bpy)Cl4], in ethylene
glycol, for 30 minutes. After filtration of non-dissolved impurities, the product was
precipitated by drop-wise addition of saturated NH4PF6(aq). After collection and drying, the
crude product was purified on an alumina column with acetonitrile as the eluent.
Electrochemical measurements were made using differential pulse polarography and
cyclic voltammetry. The following half-wave potentials were obtained for
Ru(bpy)(dpq)2](PF6)2 . The oxidation potential was +1.53 V, the first, second and third
reduction potentials were –0.66 V, -0.89 V and –1.58 V respectively, vs SSCE.
Electronic transitions were determined from the UV-Vis spectrum and summarized in
Table 1.4. The Ru (d) dpq (*) MLCT was found at 512 nm , the Ru (d)  bpy (*)
*
MLCT at 462 nm, a higher energy MLCT Ru (d)  (2 ) at 390 nm (sh) and both dpq 
* and bpy   * transitions at 281 nm and shoulders at 256 nm and 330 nm.
The excited state redox potentials were estimated from the emission spectra and
*
ground state redox potentials. Rillema et al.37 concluded that “Ru-BL 2+ complexes are poor
14
*
reductants relative to [Ru(bpy)3]+2* due to low-energy  levels but are good oxidants due to
low energy enhanced d- * interactions.”
Background on [Ru(bpy)2(dpb)](PF6)2: This compound was first synthesized by
Carlson, Wolosh and Murphy in 1991.38 The protocol for the synthesis is referenced by them
as “manuscript in preparation”, yet there is no record of this publication on record. The
comparative electrochemistry of [Ru(bpy)2(dpb)](PF6)2 and [Ru(dpb)3](PF6)2 was synthesized
and published by Carlson and Murphy in 1991.39 Brewer et al.40 published the electrochemical
properties of [Ru(bpy)2(dpb)](PF6)2 in 1992. ( see Table 1.2.)
Table 1.2. A comparison of cyclic voltammetric Data for [Ru(bpy)2(dpb)](PF6)2 (where bpy =
2,2’- bipyridine and dpb = 2,3-bis(2-pyridyl)benzo-quinaxoline).a
E1/2 (Brewer)40 (mV)
E1/2 (Murphy)39
Assignment
1766
1666
Ru(II)/Ru(III)
-334
-394
dpb/dpb-
-974
-1054
bpy/bpy-
-1314
-1374
bpy/bpy-
a
Potentials reported in CH3CN solution with 0.1 M TBAH and converted to a single NHE
scale.
Note the 50 to 100 mV discrepancy in values.
15
The electron absorption characteristics were briefly discussed by Murphy and Carlson.
They reported the Ru () dpb (*) MLCT , (at 550 nm) with a molar absorptivity of 8.2 x
103 M-1 cm-1 . Nallas and Brewer more completely correlated the UV-Vis spectra with
electronic transitions.41 (see Table 1.4).
Gordon et al 36 studied the spectroelectrochemistry of the oxidation and first reduction
of [Ru(bpy)2(dpb)](PF6)2 in 1997. Upon oxidation at 1.5V, the Ru () dpb (*) MLCT at
550 nm and Ru () bpy (*) MLCT at 425 nm (sh) were bleached and a new peak appeared
at ~ 575 nm; it was assigned as a LMCT. Upon reduction at –0.8 V, the Ru () dpb (*)
MLCT was lost and the Ru () bpy (*) MLCT was red shifted to ~ 475 nm. In comparing
the assignments of Gordon and Nallas, Gordon identified a shoulder at ~425 nm to which he
made the assignment of Ru () dpb (*) MLCT whereas Nallas did not acknowledge the
shoulder and assigned the peak at 394 nm for this transition. Gordon thought that this peak at
394 nm was a dpb   * transition. Brewer and Nallas, on the other hand , attributed this
transition to a shoulder at 362 nm, which received no assignment by Gordon.
16
Table 1.3. Summary of Electrochemical Data:
Cyclic voltammetric data for [Ru(bpy)2(dpq)](PF6)2, [Ru(bpy)2(dpb)](PF6)2,
[Ru(bpy)(dpq)2](PF6)2 and [Ru(bpy)(dpb)2](PF6)2.
COMPOUND
E1/2 (MV)
ASSIGNMENT
vs NHE
[Ru(bpy)2(dpq)](PF6)2 35
1.67
in 0.1M TEAP-CH3CN
-0.54
dpq/dpq-
at 20 +/- oC
-1.17
bpy/bpy-
Ru(II)/Ru(III)
bpy/bpy- d
[Ru(bpy)2(dpb)](PF6)2 39
1.67
in 0.1 M TBAH-CH3CN
-0.39
dpb/dpb-
-1.05
bpy/bpy-
-1.37
bpy/bpy-
[Ru(bpy)(dpq)2](PF6)2 37
1.77
Ru(II)/Ru(III)
Ru(II)/(III)
in 0.1 M TEAP-CH3CN at
-0.42
dpq/dpq-
25 +/- oC
-0.65
dpq/dpq-
-1.34
bpy/bpy-
d
An adsorption wave interfered with the determination of this potential.
17
Table 1.4 Summary of Electronic Absorption Spectroscopy:
The electronic absorption spectroscopy of [Ru(bpy)2(dpq)](PF6)2, [Ru(bpy)2(dpb)](PF6)2,
[Ru(bpy)(dpq)2](PF6)2 and [Ru(bpy)(dpb)2](PF6)2 in acetonitrile.
Compound
Wavelength
Molar Absorptivity
Assignment
(M-1 cm-1)
[Ru(bpy)2(dpq)](PF6)2 37
517 nm
8.46x103
Ru(d)dpq(*)MLCT
426 nm
8.7x103
Ru(d)bpy(*)MLCT
391,350 nm (sh)
Ru(d) (2)* MLCT
325 (sh)
  *
284 nm
7x104
  *
[Ru(bpy)(dpq)2 ](PF6)2
512 nm
9.6x103
Ru(d)dpq(*)MLCT
37
462 nm
8.3x103
Ru(d)bpy((*)MLCT
[Ru(bpy)2(dpb) ](PF6)2
36
390 nm (sh)
Ru(d) (2*) MLCT
256,330 nm (sh)
  *
281 nm
6.9x104
  *
550 nm
8.3x103
Ru(d)dpb(*)MLCT
425 nm (sh)
Ru(d)bpy (*)MLCT
392 nm(sh)
dpb   *
314 nm
4.5x104
285 nm
6.8x104
bpy   *
242 nm
4.5x104
  *
[Ru(bpy)2(dpb) ](PF6)2
550 nm
9.5x103
Ru(d)dpb(*)MLCT
41
394 nm
1.4x 104
Ru(d)bpy (*)MLCT
dpb   *
362 nm(sh)
286 nm
7.5x104
bpy   *
18
Statement of Purpose
In summary, the goal of this work is to probe the electrochemical, electron absorption
spectroscopic and spectroelectrochemical properties of ruthenium(II) complexes with
polyazine bridging ligands, focusing on the dpb ligand. This involved the preparation and
study of the new complex [Ru(bpy)(dpb)2](PF6)2 as well as the previously reported
compounds [Ru(bpy)(dpq)2](PF6)2 , [Ru(bpy)2(dpb)](PF6)2 and [Ru(bpy)2(dpq)](PF6)2.
19
References
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2.
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Roundhill, D. M. Op. Cit., 175.
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Demas, J. N.; Adamson, A. W. J. Am. Chem. Soc. 1971, 93, 1800.
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Kalyanasundaram, K. in Photosensitization and Photocatalysis Using Inorganic and
Organometallic Compounds, Ed. By Kalyanasundaram, K.; Gratzel, M., 1993, 113,
Kluwer, Norwell MA.
22.
Balzani, V.; Maestri, M. in. Photosensitization and Photocatalysis Using Inorganic and
Organometallic Compounds, Ed. By Kalyanasundaram, K.; Gratzel, M., 1993, 33,
Kluwer, Norwell MA.
23.
Kalyanasundaram, Op. Cit., 161.
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Kalyanasundaram, Op. Cit., 142.
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Moggi, L. In Photoelectrochemistry, Photocatalysis and Photoreactors, Ed.
Schiavello.M. Nato ASI Series.1984, 197-216, Reidel, Boston.
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Richter, M. M.; Brewer, K. J. Inorg.Chem. 1993, 32, 5762.
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Szabo, E. G.; Murphy, W. R. Inorg. Chem. 1990,29, 2327. (b) Stephen , W. I. And
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22
CHAPTER TWO: EXPERIMENTAL METHODS
Materials
HPLC grade acetonitrile was purchased from Mallinckrodt. It was further purified by
distillation over CaCl2 chips in a N2 atmosphere. RuCl3 3H2O was procured from Johnson
Matthey/Alfa Aesar . The terminal ligand 2,2’-bipyridine was purchased from Aldrich.
[Ru(bpy)2(dpq)](PF6)2 was synthesized by Carla Wibble using a modified synthetic procedure
of Rillema and Mack.1 [Ru(bpy)(dpq)2](PF6)2 was synthesized by Carla Wibble using a
modified synthetic procedure of Rillema et al.2
Electronic Absorption Spectroscopy
UV-Vis spectra were recorded at room temperature with A HP 8452A diode array
spectrophotometer having 2 nm resolution, using 1cm path length quartz cuvettes. Samples
were dissolved in acetonitrile.
Cyclic Voltammetry and Osteryoung Square Wave Voltammetry
All electrochemistry was performed using a Bio Analytical Systems 100 W potentiostat and
accompanying software. Analytes were dissolved in acetonitrile containing 0.1M TBAH. A
three-electrode system was used, consisting of a BAS Pt wire secondary electrode, BAS epoxy
impregnated 1.6 mm Pt working electrode and a BAS Ag/AgCl aqueous reference electrode
which was calibrated with ferrocene. The ferrocene to ferrocenium oxidation was used as a
reference of 660 mV vs NHE.3
23
Spectroelectrochemistry, Bulk (H-cell) Construction and Operation
The H-cell consisted of a 1 cm quartz cuvette (vertical) fused to a horizontal glass tube with a
medium glass frit (constructed by the glass shop)(Figure 2.1).4 Adjoining the horizontal
connecting portion was another vertical glass compartment. The platinum wire counter
electrode was immersed in the vertical glass compartment which was filled with solvent and
supporting electrolyte. It may contain the analyte to minimize a concentration gradient. The
working electrode, which consisted of a platinum gauze, was immersed in the quartz cuvette
which contained solvent (acetonitrile), TBAH (supporting electrolyte) and analyte. The
reference electrode was positioned above the working electrode and was separated by a
medium porosity glass frit. The solution in the quartz cuvette was mixed and deoxygenated by
bubbling with argon. The potentials for oxidation and reduction were determined by cyclic
voltammetry. The potentials chosen were more negative (for reduction) by approximately 0.2
V and around 0.2V more positive for oxidation. Prior to each spectral acquisition, the argon
was turned off. Initially the ground state UV-Vis spectra was taken at 0 V. The potential was
then changed and periodic spectra were taken. The current display on the potentiostat
decreased as the amount of the electrochemically generated product increased ( which implies
that the amount of the starting material decreased. The completion of each electrochemical
step was determined by both a lack of change in spectra and diminished current. Experience
showed that the less concentrated the original solution, the less the analysis time (very
profound), therefore very dilute solutions were used. After each electrochemical step, the
reversibility of that step was tested by returning to the initial potential. The degree of
24
reversibility was gauged by the degree of superimposibility between the original and last
spectrum.
Figure 2.1. H-cell for bulk spectroelectrochemistry.
Optically transparent thin layer cell
The design for the OTTLE cell was a slightly-modified version of Krejcik et al. (Figure 2.2).5
The cell utilized a Perkins-Elmer demountable IR cell. For UV-Vis spectra, quartz windows
were substituted for salt optical windows. A three electrode system was sandwiched between
two sheets of 6mm polyethylene and fused together with a thermostated hydraulic press. The
pseudo-reference was made from three strands of 0.05 mm silver wire wrapped together. The
counter electrode was made from a piece of Pt gauze. The working electrode was made from a
100 line/inch gold cloth (Buckbee Mears). Silver paint (Aldrich) was used to join 0.10 mm
silver leads to the appropriate electrode. The cell has been modified by extending the
polyethylene sheets to where there is a dam with ~1 mm opening between the working and
counter electrode. A specially designed platform was built (by physics machine shop) to screw
25
into the sample-holder base and hold the OTTLE cell so that the optical window and beam
path were coincident.
(A) Schematic diagram of the IR OTTLE cell with the three electrode
system placed inside the thin layer. (a) Steel pressure plate; (b) inlet; (c)
Teflon gasket; (d,f) front and back KBr windows; (e) polyethylene
spacer with melt-sealed electrodes; (g) Teflon holder; (h) back plate.
(B) Front view of the cell composed of the parts (e)-(h) of (A) showing
the position of the electrodes and their soldered contacts with Cu
conductors fixed by screws. (a) Au minigrid working electrode with
twinned contact Ag wire; (b) Pt mesh auxiliary electrode; (c) Ag wire
pseudoreference electrode.
Figure 2.2. The OTTLE cell of Krejcik et al.5
Reprinted from Journal of Electroanalytical Chemistry, Vol 317, Krejcik, M.; Danek, M.;
Hartl, J., pp179, 1991, with permission of Elsevier Science”.
26
Prior to mounting the OTTLE cell in the platform cell, the blank sample (0.1M TBAH
in deoxygenated CH3CN) was syringed into the cell via one Luer port. Air and excess solution
came out from the opposite port. Once free of air bubbles, the ports were capped. The cell was
mounted and appropriate electrode leads were connected. The short optical path length (~0.17
mm) of the OTTLE cell necessitated a more concentrated solution as compared to the H-cell.
Due to the small volume and large ratio of the surface area of the working electrode to
volume, the analysis time was greatly shortened. Complete reductions or oxidations were
completed in one to three minutes. The experimental procedures for the spectral acquisition of
spectroelectrochemical data was similar to the protocol for the H-cell.
Mass Spectroscopy
Mass spectra were determined with a Fisions Instruments VG Quattro Triple Stage
Quadrapole Mass Spectrometer. Although commonly called Fast Atom Bombardment, in
positive ion mode, (FAB+), Liquid Secondary Ionic Mass Spectroscopy (LSIMS) is a more
accurate acronym. The sample molecules, dissolved in a matrix of glycerol or nitro- benzyl
alcohol, are ionized by collision with a beam of cesium ions which are accelerated by a 20 kV
bias.
Synthesis
TBAH
Tetrabutyammoniumhexafluorophosphate (TBAH). Single batches were made by
combining Bu4NBr (Aldrich) (50.0g, 231 mmol in 600 mL deionized water) and KPF6
(Aldrich) ( 31.0g, 168 mmol in 50 mL DI water). The precipitate, TBAH(s), was separated
27
from KBr(aq) (supernatant) by vacuum filtration. The TBAH was then dissolved in 800 ml hot
ethanol. DI water was then added to make a saturated solution (~400ML). The solution was
cooled to facilitate crystallization. Two to three additional recrystallizations were performed.
o
The product was vacuum dried at ~87 C and analyzed by cyclic voltammetry for purity.
Figure 2.3 shows a representative CV. Note that there is no superoxide peak at ~-0.8 V and no
reduction of hydrogen (from water contamination) at the –1.8 V.
Figure 2.3. Background CV of 0.1 M TBAH in acetonitrile at room temperature using a
Ag/AgCl reference electrode.
2,3-bis(2-pyridyl)quinoxaline (dpq)
The synthesis was a modification of Goodwin and Lions 6 . O-phenylenediamine
(Aldrich)(2.16 g, 20.0 mmol) and 2,2’-pyridil (4.24 g, 20.0 mol) were dissolved in 50 ml
100% ethanol and heated for 2 hours. The product was recrystallized 5 times in 100% ethanol.
28
2,3-bis(2-pyridyl)benzoquinoxaline (dpb)
The synthesis was a modification of Goodwin and Lions.
6
2,3-diaminonapthalene (2.498 g,
15.78 mmol) and 2,2’-pyridil (3.393 g, 15.98 mmol) were dissolved in 125 ml 100% ethanol
and refluxed for 5 hours and cooled overnight.
The precipitate was recrystallized in ~30 ml 100% ethanol and dried by evaporation. Yield
was 2.1 g or 41%. The product was further purified on an alumina column with methylene
chloride as eluent. The final purified product was analyzed for purity by absence of C=O
stretch from 1690 to 1760 cm-1 in IR spectra. Analysis by cyclic voltammetry and UV-Vis
spectroscopy were also performed.
Bis(2,2’-bipyridine) ruthenium(II) dichloride. [Ru(bpy)2Cl2]
7
The synthesis was according to Sullivan et al. RuCl33H2O (4.73 g, 180 mmol), 2,2’-pyridyl
(bpy)(5.62 g, 36.0 mmol), LiCl (3.79 g, 90.0 mmol) were dissolved in 35 mL DMF and heated
for 7.5 hours. After addition of a liberal amount of acetone, the solution was cooled in the
refrigerator overnight. The product was then isolated by vacuum filtration with a glass frit.
The filtrate was washed with a copious amount of DI water and diethyl ether and then dried in
a dessicator with CaH2 chips.
2,2’-bipyridine ruthenium(II) tetrachloride.[Ru(bpy)Cl4]
The synthesis was described by Krause.8 RuCl33H2O (5.15g, 19.7 mmol) and 2,2’-bipyridyl
(3.75g, 24.0 mmol) were combined with 25 ml 1N HCl . The components were left in covered
29
beaker for ~2.5 months. The product was collected by vacuum filtration with a glass frit and
dried over P2O5. The product was analyzed by LSIMS. See Appendix Figure 3.8.
Bis(2,2’-bipyridine)2,3-bis(2’-pyridyl)benzoquinoxaline)ruthenium(II) Hexafluorophosphate.
[Ru(bpy)2(dpb)](PF6)2
Ru(bpy)2Cl2 (0.258 g, 0.532 mmol) and
dpb (0.174 g, 1.11 mmol) were dissolved in 30 mL ethanol and 15 mL DI water and refluxed
for 2.5 hours. The product was precipitated by the addition of 100 mL saturated KPF6 and
isolated by vacuum filtration with a glass frit. Product was washed with ethanol and diethyl
ether. Final product was dried in a dessicator for 2 days. Yield was 0.486 g or 88.1% .
(2,2’-bipyridine)(2,3-bis(2-pyridyl)quinoxaline)ruthenium(II) Hexafluorophosphate.
[Ru(bpy)(dpq)2](PF6)2:
This synthesis is a modification of the literature synthesis of Rillema et al.2 Ru(bpy)Cl4 (0.223
g, 0.853 mmol) and dpq (0.382 g, 1.34 mmol) were combined with 30 ml 95% ethanol and
heated for ~ 5 days under argon. An 80 ml saturated solution of KPF6 was diluted with 40 ml
DI water and added to the reacted solution. The mixture was cooled in the refrigerator and
stored therein for further purification.
(2,2’-bipyridine)(2,3-bis(2-pyridyl)benzoquinoxaline)ruthenium(II) Hexafluorophosphate.
[Ru(bpy)(dpb)2](PF6)2
Ru(bpy)Cl4 (0.209 g,mmol) and dpb (0.345g, 1.03 mmol) were combined with 40mL 95%
ethanol and 15 ml DI water. This as heated under water, argon for ca. 5 days. An 80 ml
saturated solution of KPF6 was diluted with 40 ml DI which was then added to the round
30
bottom flask and refrigerated for several days. The products were collected by vacuum
filtration, with a glass frit and dried with ca. 150 ml diethyl ether. Ethanol was mistakenly
added to the drying product which resulted in some loss of product as indicated by the eluent
changing color from clear to purple. The dried product was analyzed with LSIMS (see Figure
3.4) and purified by alumina chromatography. The dpb (yellow band) was first eluted using
1:2, acetonitrile/toluene . The remaining purple band was removed from the column and added
as a slurry on a shorter column to speed purification, using 2:1 acetonitrile/toluene. Two bands
were collected, the first, dark purple and the second, dark pink. The first band was evaporated
to precipitate the product. This product was dissolved in a minimal amount of acetonitrile and
run through another alumina column (smaller bore, shorter length) with 2:1
acetonitrile/toluene mobile phase. From this dark purple band, three eluents were collected,
evaporated under vacuum and dissolved in pure acetonitrile. The final product (actually three )
was flash precipitated by addition of diethyl ether, filtered and dried.
31
References
1.
Rillema, D. P.; Mack, K. B. Inorg. Chem. 1982, 21, 3849.
2.
Rillema, D. P.; Taghdiri, D. G.; Jones, D. S.; Keller, C. D.; Worl, L. A.; Meyer, T. J.;
Levy, H. A. Inorg. Chem. 1987, 26, 578.
3.
Gagne, R. R.; Koval, C. A.; Lisensky, G. C. Inorg. Chem. 1980, 19, 2855.
4.
Jones, S. W. Ph.D. Thesis, p 50. Virginia Tech, 1999.
5.
Krejcik, M.; Danek, M.; Hartl, J. J. Electroanal. Chem. 1991, 317, 179.
6.
Goodwin, H. A.; Lions, F. J. Am. Chem. Soc. 1959, 81, 6415.
7.
Sullivan, B. P.; Salmon, D. J.; Meyer, T, J. Inorg. Chem. 1978, 17, 3334.
8.
Krause, R. A. Inorgica Chimica Acta. 1977, 22, 209.
32
CHAPTER THREE: RESULTS
The four ruthenium polyazine bridging ligand complexes were prepared according to a
similar two step synthetic strategy whereby (1) the terminal ligand, bpy, was first bound to the
ruthenium metal center and (2) the bridging ligand was bound to the metal center of the
intermediate. After purification on neutral alumina, compounds were analyzed for identity and
purity.
Identification using Liquid Secondary Ion Mass Spectroscopy
Elemental analysis was not performed on our polyazine complexes of ruthenium (II)
because of its inability to clearly distinguish monometallic mixed ligand complexes and
bimetallic complexes.1 For example, when comparing [Ru(bpy)2(dpq)](PF6)2 and
{[Ru(bpy)2]2(dpq)}(PF6)4 , the % (N / total N, C, H) is 18.8 % and 18.7% respectively. The %
(C/ total C, N, H) is 56 % vs 54 %. For H the % is 5 % vs 6 % . Furthermore, solvents of
crystallization are a common complicating factor.
Instead, identifications were determined with FAB+-MS (more precisely LSIMS)
Accurate mass spectral interpretation requires that the LSIMS of matrices, starting materials
and intermediates be known. The LSIMS of the two matrices used, glycerol (MW 92+1) and
nitro-benzyl alcohol (MW 153+1) are seen in Appendix Figures 3.1 and 3.2. The LSIMS of
the free ligands bpy, dpq and dpb are shown Appendix Figures 3.3, 3.4, 3.5. The LSIMS of
[Ru(bpy)Cl4] (MW 399) , as seen in Appendix Figure 3.6, does not indicate that the molecular
ion is present. In fact, peaks at 449 m/z and 414 m/z could be [Ru(bpy)2Cl]+ and [Ru(bpy)2]+
respectively. Note the peak at 207 m/z, this may be evidence of the unreacted reactant RuCl3
minus the three waters of hydration. The mass spectrum of the final product
33
[Ru(bpy)2(dpb)](PF6)2 (MW 1038), is shown in Figure 3.1. The peak at 892 m/z is
[Ru(bpy)2(dpb)](PF6)+ . The peak at 750 m/z represents [Ru(bpy)2(dpb)]+. The mass spectrum
of [Ru(bpy)2(dpq)](PF6)2 ( MW 988) is shown in Figure 3.2. The mass peak at 842 m/z is the
[Ru(bpy)2(dpq)](PF6)+ cation, the peak at 695 m/z is [Ru(bpy)2(dpq)]+ . The peak at 412 m/z
represents [Ru(bpy)2]+. The mass spectrum of [Ru(bpy)(dpq)2](PF6)2 (MW 1117) is seen in
Figure 3.3. The peaks at 973 m/z and 826 m/z are the cations [Ru(bpy)(dpq)2](PF6)+ and
[Ru(bpy)(dpq)2]+ respectively. The LSIMS+ mass spectrum of [Ru(bpy)(dpb)2](PF6)2 (MW
1217) is displayed in Figure 3.4. The mass peaks at 1071 m/z and 926 m/z are the cations
[Ru(bpy)(dpb)2](PF6)+ and [Ru(bpy)(dpb)2]+ respectively. The mass peak centered at 1104 m/z
may implicate a chloride adduct.
34
Figure 3.1. LSIMS of [Ru(bpy)2(dpb)](PF6)2
35
Figure 3.2. LSIMS of [Ru(bpy)2(dpq)](PF6)2.
36
Figure 3.3. LSIMS of [Ru(bpy)2(dpq)](PF6)2.
37
Figure 3.4. LSIMS of [Ru(bpy)(dpb)2](PF6)2 .
Electrochemistry of Background Electrolyte and Ligands
Accurate data acquisition with the three electrode electrochemical system requires a
good background provided by pure electrolyte (TBAH) and both dry and well deoxygenated
solvent (acetonitrile). Figures 2.3, 3.5, and 3.6 are representative CV and OSWV of the
solvent electrolyte system. For purposes of comparison, the Ag/AgCl reference electrode is
standardized vs ferrocene/ferrocenium which is known to be 665 mv vs NHE .1 Additionally,
this well established, fully reversible one electron oxidation/reduction can be used as a
anodic
reference for Nernstian reversibility. Theoretically Ep, (where Ep= Ep
cathodic

–Ep
) should be 59 mV for a reversible one electron transfer.2 Using resistance compensation, an 
Ep of 56 mV was obtained with a platinum electrode (Figure 3.7) was achieved .3 Figure 3.8
38
shows how a highly concentrated solution gives an exaggerated peak separation of 462 mV.
(IR compensation will not eliminate this exaggerated peak separation).
Figure 3.5. OSWV of background electrolyte, 0 to –1.8 V. 0.1 M TBAH in acetonitrile.
39
Figure 3.6. OSWV of background electrolyte, 0 to 1.8 V. 0.1 M TBAH, in acetonitrile.
Figure 3.7. CV of ferrocene oxidation with IR (internal resistance) compensation and
platinum electrode in acetonitrile. Ep = 56 mV.
40
Figure 3.8. CV of a high concentration of ferrocene with no IR compensation in
acetonitrile. Ep = 462 mV
41
Electrochemistry-OSWV
The Osteryoung square wave voltammagram of [Ru(bpy)2(dpb)] (PF6)2 is shown in
Figure 3.9. The OSWV of [Ru(bpy)2dpq)](PF6)2 shown in Figure 3.10. The OSWV of
[Ru(bpy)(dpq)2] (PF6)2 is seen in Figure 3.11. The OSWV of [Ru(bpy)(dpb)2](PF6)2 Figure
3.12. The OSWV of [Ru(bpy)3](PF6)2 as seen in Figure 3.13.
Figure 3.9. Osteryoung square wave voltammogram of [Ru(bpy)2(dpb)](PF6)2 (where
bpy= 2,2’ bipyridine, dpb= 2,3-bis(2-pyridyl)benzoquinoxaline and using a Ag/AgCl
reference electrode).
42
Figure 3.10. Osteryoung square wave voltammogram of [Ru(bpy)2(dpq)](PF6)2 (where
bpy= 2,2’ bipyridine, dpq= 2,3-bis(2-pyridyl)quinoxaline and using a Ag/AgCl reference
electrode).
43
Figure 3.11. Osteryoung square wave voltammogram of [Ru(bpy)(dpq)2](PF6)2 (where
bpy= 2,2’ bipyridine, dpq= 2,3-bis(2-pyridyl)quinoxaline and using a Ag/AgCl reference
electrode).
44
dpb0/-1
dpb0/-1
bpy0/-1
RuII/II
I
Figure 3.12. Osteryoung square wave voltammogram of [Ru(bpy)(dpb)2](PF6)2 (where
bpy= 2,2’ bipyridine, dpb = 2,3-bis(2-pyridyl)benzoquinoxaline and using a Ag/AgCl
reference electrode).
45
bpy0/
bpy0/
-1
-1
bpy0/
-1
bb
Figure 3.13. Osteryoung square wave voltammogram of [Ru(bpy)3](PF6)2 (where bpy =
2,2’ bipyridine and using a Ag/AgCl reference electrode).
Electrochemistry-Cyclic Voltammetry
The CV’s of [Ru(bpy)2(dpq)](PF6)2, [Ru(bpy)2dpb)](PF6)2 and Ru(bpy)(dpq)2](PF6)2
are shown in Figures 3.14, 3.15 and 3.16. The CV of [Ru(bpy)(dpb)2](PF6)2 is shown in
Figure 3.17. Table 3.1 summarizes the oxidative and reductive potentials, relative to NHE, of
the compounds [Ru(bpy)2(dpq)] (PF6)2, [Ru(bpy)2(dpb)](PF6)2, [Ru(bpy)(dpq)2](PF6)2 , and
[Ru(bpy)(dpb)2](PF6)2 .
46
Table 3.1. Cyclic voltammetric data for a series of ruthenium (II) complexes incorporating
polypyridyl ligands ( where bpy = 2,2’-bipyridine, dpq = 2,3-bis(2-pyridyl)quinoxaline , dpb
=2,3-bis(2-pyridyl)benzoquinoxaline).a
Compound
E1/2 oxid
E1/2 red(1)
E1/2 red(2)
E1/2 red (3)
1700
-510
-1150
-1370
1760
-330
-1020
-1410
1850
-410
-650
-1350
~1900
-250
-470
~-1280
[Ru(bpy)3 ] (PF6)2
1600
-1010
-1200
-1430
[Ru(dpb)3](PF6)2 1
1910
-240
-410
-640
[Ru(bpy)2(dpq)]
(PF6)2
[Ru(bpy)2(dpb)]
(PF6)2
[Ru(bpy)(dpq)2]
(PF6)2
[Ru(bpy)(dpb)2]
(PF6)2
a
Potentials measured in CH3CN solution with 0.1 M TBAH and Ag/AgCl reference electrode
with a scan rate of 200 mV/sec and converted to a NHE scale.
47
Figure 3.14. Cyclic voltammogram of [Ru(bpy)2(dpq)](PF6)2 (where bpy= 2,2’bipyridine, dpq = 2,3-bis(2-pyridyl)quinoxaline and using a Ag/AgCl reference electrode
with a scan rate of 200 mV/sec).
Figure 3.15. Cyclic voltammogram of [Ru(bpy)2(dpb)](PF6)2 (where bpy= 2,2’bipyridine, dpb = 2,3-bis(2-pyridyl)benzoquinoxaline and using a Ag/AgCl reference
electrode with a scan rate of 200 mV/sec).
48
Figure 3.16. Cyclic voltammogram of [Ru(bpy)(dpq)2](PF6)2 (where bpy= 2,2’bipyridine, dpq = 2,3-bis(2-pyridyl)quinoxaline and using a Ag/AgCl reference electrode
with a scan rate of 200 mV/sec).
Figure 3.17. Cyclic voltammogram of [Ru(bpy)(dpb)2](PF6)2 (where bpy = 2,2’bipyridine, dpb = 2,3-bis(2-pyridyl)benzoquinoxaline and using a Ag/AgCl reference
electrode with a scan rate of 200 mV/sec).
49
UV-Vis Spectroscopy
Ligands- The UV-Vis electronic absorption spectra of the ligands bpy, dpq and dpb are
shown in Figure 3.18 (molar absorptivity vs wavelength).
Figure 3.18. The comparative UV-Vis spectra (molar absorptivity vs wavelength) of the
ligands bpy (….), (where bpy = 2,2’-bipyridine) dpq (___), (where dpq =2,3-bis(2pyridyl)quinoxaline) and dpb (_ _ _), (where dpb =2,3-bis(2-pyridyl)benzoquinoxaline).
50
Monometallic Ruthenium Complexes
The UV-Vis spectra of [Ru(bpy)2(dpq)](PF6)2, [Ru(bpy)2(dpb)](PF6)2,
[Ru(bpy)(dpq)2](PF6)2 and [Ru(bpy)(dpb)2](PF6)2 are shown in Figure 3.19.
Figure 3.19. The comparative electronic absorption spectra of [Ru(bpy)2dpq](PF6)2
(light blue), [Ru(bpy)2(dpb)](PF6)2 (dark blue), [Ru(bpy)(dpq)2](PF6)2 (black) and
[Ru(bpy)(dpb)2](PF6)2 (red) in acetonitrile at room temperature. (Where bpy = 2,2’bipyridine, dpq = 2,3-bis(2-pyridyl)quinoxaline) and dpb = 2,3-bis(2-pyridyl)
benzoquinoxaline).
51
Spectroelectrochemistry
Initially, spectroelectrochemical data were obtained using an H-cell apparatus. See
Figure 2.1. The difficulty in obtaining reversible oxidations from compounds previously
reported to display reversible behavior prompted the employment of an OTTLE cell.4 See
Figure 2.2. As ferrocene/ferrocenium has served as the model, one-electron reversible
oxidation, the spectra of its electrochemical oxidation (1.71V) and reduction
(-0.25V) to the original species are shown in Appendix Figure 3.7.
[Ru(bpy)2(dpq)](PF6)2
The oxidation (at 1.89 V) and regeneration of the parent complex (at –0.25 V) with the
H-cell are shown in Figure 3.20. The oxidative spectra obtained with the OTTLE cell are seen
in Figure 3.21. The one electron reduction of the complex at –1.00 V lead to irreversible
behavior with the H-cell, (see Figure 3.22). The reductive process was reversible with the
OTTLE cell as seen in Figure 3.23. The graph of the first, second and third reductions is
shown in Figure 3.24. The second and third reductions were not reversible.
52
Figure 3.20. Oxidative spectroelectrochemistry of [Ru(bpy)2(dpq)](PF6)2 with H-cell.
(___= 0V, _ _ _ = 1.89V, ….. = -0.25V). (Where bpy = 2,2’-bipyridine, dpq = 2,3-bis(2pyridyl)quinoxaline). 0.1M TBAH in CH3CN at room temperature.
53
Figure 3.21. Oxidative spectroelectrochemistry of [Ru(bpy)2(dpq)](PF6)2 with OTTLE
cell. (Black = 0V, red = 1.90V, blue = 0V). (Where bpy = 2,2’-bipyridine, dpq = 2,3-bis(2pyridyl)quinoxaline). 0.1M TBAH in CH3CN at room temperature.
54
Figure 3.22. First reduction of [Ru(bpy)2(dpq)](PF6)2 with H-cell. (___ = 0V, _ _ _=
-1.00V, …. = 0.25V). (Where bpy= 2,2’-bipyridine, dpq = 2,3-bis(2-pyridyl)quinoxaline).
0.1M TBAH in CH3CN at room temperature.
55
Figure 3.23. First reduction of [Ru(bpy)2(dpq)](PF6)2 with OTTLE cell. (Black = 0V, red
= -1.00V, blue = 0V). (Where bpy = 2,2’-bipyridine, dpq = 2,3-bis(2pyridyl)quinoxaline). 0.1 M TBAH in CH3CN at room temperature.
56
Figure 3.24. First, second and third reduction of [Ru(bpy)2(dpq)](PF6)2 with the OTTLE
cell. (Where bpy = 2,2’-bipyridine, dpq = 2,3-bis(2-pyridyl)quinoxaline). (Black = 1.00V, red = -1.55V, blue = 1.90V). 0.1 M TBAH in CH3CN at room temperature.
57
[Ru(bpy)(dpq)2](PF6)2
The oxidation (1.80 V) and reduction to generate the parent complex seen in Figure
3.25. The first reduction (-0.70 V) and oxidation to generate the parent complex are shown in
Figure 3.26. The reduction is not fully reversible. Further reduction ( -1.00V) leads to loss of
reversibility. See Figure 3.27.
Figure 3.25. Oxidative spectroelectrochemistry of [Ru(bpy)(dpq)2](PF6)2 with OTTLE
cell. (Where bpy= 2,2’-bipyridine, dpq = 2,3-bis(2-pyridyl)quinoxaline). (Black = 0 V,
red = 1.80 V, blue = 0 V). 0.1 M TBAH in CH3CN at room temperature.
58
Figure 3.26. First reductive spectroelectrochemistry of [Ru(bpy)(dpq)2](PF6)2 with
OTTLE cell. (Where bpy = 2,2’-bipyridine, dpq = 2,3-bis(2-pyridyl)quinoxaline). (Black
= 0 V, red = -0.70 V, blue = 0V). 0.1 M TBAH in CH3CN at room temperature.
59
Figure 3.27. First and second reductive spectroelectrochemistry of
[Ru(bpy)(dpq)2](PF6)2 with OTTLE cell. (Where bpy = 2,2’-bipyridine, dpq = 2,3-bis(2pyridyl)quinoxaline). Black = 0 V, red = -0.70 V, blue = -1.00 V. 0.1 M TBAH in CH3CN
at room temperature.
60
[Ru(bpy)(dpb)2](PF6)2
The spectral data for this compound were acquired with the OTTLE cell. The
oxidation at 1.75 V and reduction to the parent compound are shown in Figure 3.28. The
reduction at –0.50 V and oxidation to the parent compound are seen in Figure 3.29. The
composite of subsequent reductions at –0.60 V, -1.00 V and –1.75 V is shown in Figure 3.30.
The reductions at –1.00 V and –1.75 V were irreversible.
Figure 3.28. The oxidative spectroelectrochemistry of [Ru(bpy)(dpb)2](PF6)2
with the OTTLE cell. (Black = 0 V, red = 1.75 V, blue = 0 V). 0.1M TBAH in CH3CN at
room temperature.
61
Figure 3.29. The first reductive spectroelectrochemistry of [Ru(bpy)(dpb)2](PF6)2 with
the OTTLE cell. ( Solid line = 0 V, dash and dots = -0.50 V, dots = 0 V). 0.1 M TBAH in
CH3CN at room temperature.
62
Figure 3.30. Composite of parent complex, first, second and third reduction
spectroelectrochemistry of [Ru(bpy)(dpb)2] (PF6)2 with the OTTLE cell.( Black = 0 V,
red = -0.50 V, blue =-1.00 V, light blue = -1.75 V). 0.1 M TBAH in CH3CN at room
temperature.
63
References
1.
Carlson, D. L.; Murphy, W. R. Inorg. Chim. Acta. 1991, 181, 61.
64
CHAPTER FOUR: DISCUSSION
Synthesis and Purification
Monoruthenium mixed polypyridyl complexes can be prepared by two different
synthetic strategies. (1) The bridging ligand is combined with RuCl33H20 in ethanol/water to
yield [Ru(BL)xCl 6-2x]. This yield is often low due to the formation of the polymeric systems.
To minimize this, the bridging ligands can be protected on one side and deprotected after
binding to the metal.1 In either case the terminal ligand bpy is then added via a substitution for
the remaining chlorides. (2) The somewhat simpler synthetic route is to add the terminal
ligand bpy to the RuCl33H20 and then add the bridging ligand. This is the chosen method for
the complexes synthesized in this study. Purification by adsorption chromatography on
alumina was effective in removing impurities (unreacted starting materials, bimetallic and
polymetallic complexes, and complexes with unwanted BL/bpy stoichiometries) for
[Ru(bpy)2(dpq)](PF6)2 , [Ru(bpy)(dpq)2](PF6)2 , and [Ru(bpy)2(dpb)](PF6)2. Purification was
less effective for the new compound [Ru(bpy)(dpb)2](PF6)2. After two alumina
chromatographs, there remained ca 1% electroactive impurities as determined by OSWV.
The major difference between the protocol used by the Brewer3 group and Rillema,2
for the synthesis of the ruthenium dpq complexes, was the choice of solvent. The Brewer
protocol uses an ethanol/ water solvent mixture, whereas Rillema used acetone (for the
synthesis of [Ru(bpy)2(dpq)](PF6)2) and ethylene glycol (for the synthesis of
[Ru(bpy)(dpq)2](PF6)2). Additionally, Rillema removed the chlorides from [Ru(bpy)Cl4] with
the addition of AgPF6 prior to the addition of the dpq ligand.
65
Identification of Products
As illustrated in the results section, page 35, elemental analysis is not an effective
method of identification for these types of complexes. Liquid secondary ion mass
spectroscopy (LSIMS),on the other hand, will accurately confirm the identity of the parent ion.
(Most ions formed will have +1 charge). LSIMS will not determine the purity of the
compound since these complexes fragment extensively due to collisions with accelerated
cesium atoms.
The mass spectra of all compounds analyzed herein show the loss of one PF6counterion. The relative intensity of the parent ion minus one PF6 - is not of diagnostic
importance, as most of the sample is fragmented into lower mass ions. The crucial factor is
that the parent ion is present. The LSIMS results, pages 37-40, confirm the identity of the four
synthesized compounds.
Determination of Purity
Osteryoung square wave voltammetry is a sensitive method for detecting electroactive
impurities.4 It is well known that the observed oxidation for each of the four compounds being
studied represents the Ru II/III couple. Typically, reductions are ligand centered and bridging
ligands (dpq and dpb) reduce before the terminal ligand bpy.
The OSWVs of the following three complexes [Ru(bpy)2(dpb)](PF6)2 (as seen in
Figure 3.9), [Ru(bpy)2(dpq)](PF6)2 (as seen in Figure 3.10) and Ru(bpy)(dpq)2](PF6)2,
( as seen in Figure 3.11), show no detectable electroactive impurities. Figure 3.12 is the
OSWV of the same compound (synthesized by the author). The small peak at 1.9 V may be
evidence water. The small shoulder at –1.6 V is indicative of a small amount of polypyridyl
66
impurity. For the sake of comparison, the OSWV of [Ru(bpy)3](PF6)2 is seen in Figure 3.13.
The compound shows no detectable impurities. It is interesting that whereas
[Ru(bpy)(dpq)2](PF6)2 was synthesized and purified satisfactorily, [Ru(bpy)(dpb)2](PF6)2 was
not. This can be explained both in terms of increased steric hindrance and decreased sigma
donation of the dpb ligand.2c
Curiously, the level of purity in [Ru(bpy)(dpb)2](PF6)2 decreased with more than two
runs through neutral alumina . This would suggest that there may be some degradation of the
product due to interaction with the neutral alumina or an increased retention of the desired
product. Silica and Florisil magnesium hydrogen silicate) were also tried but the compounds
irreversibly adsorbed to these adsorbents. This dilemma begged the following question. Could
the starting materials and intermediate compounds be further purified?
Cyclic Voltammetry
The monometallic ruthenium polypyridyl complexes are known to undergo a one
electron, metal based oxidation and multiple ligand based reductions.5 The potential of the
first reduction reflects the relative energy of the lowest lying, most stable * ligand molecular
orbital (MO). The ease of reduction for the free ligands (least negative E1/2 vs NHE) is dpb
(-0.85 V) >dpq (-1.14 V) >bpy(-1.99 V). The ratio of peak currents (Ipa/Ipc  and the symmetry
of the reductive and oxidative peaks indicates reversible electron transfer. The Epa- Epc  59
mV rule of thumb for Nernstian behavior suggests a one electron transfer.6
Many factors impact the signal to noise ratio and detectability of impurities in
ruthenium complexes. The cyclic voltammogram varies with the purity of the compound and
the purity of the solvent/electrolyte system. Water contamination can lead to a lower solvent
67
window and can obscure the third reduction peak for systems of this type. The presence of
oxygen will introduce an irreversible reduction peak at ~ -1.0V which represents the
reduction of oxygen to superoxide radical. The cyclic voltammogram of
[Ru(bpy)(dpb)2](PF6)2 shows more impurity and a narrower solvent window as compared to
the other compounds.
A comparison between the reductive peaks of the free ligands and the first reductive
peaks of the complexes ( Table 4.1 ) reveals the stabilization of the ligand * upon
coordination to the ruthenium metal center. The ease of reduction of the ligand (less negative
E1/2) follows the same order as in the ruthenium complexes (dpb>dpq>bpy). The substantial
stabilization of the ligands upon complexation, implies a decrease in electron density due to
sigma donation to the metal center.7 When comparing mono vs bis- bridging ligands
complexes (dpq and dpb) the bis-bridging ligand complex is always easier to reduce by about
80 mV. This effect may be rationalized by the decrease in -donation (from bis-bpy vs monobpy) to Ru which stabilizes the metal center and consequently stabilizes the bridging ligand.
Alternatively, the presence of two bridging ligands gives two systems with enhanced metal to
ligand backbonding to be shared thereby reducing the destabilization on each bridging ligand.
68
Table 4.1. Reduction potentials vs NHE in acetonitrile with 0.1 M TBAH at room
temperature for a series of polypyridyl ligands and ruthenium complexes.
Complex or ligand
E1/2 (mV)
[Ru(bpy)2(dpq)](PF6)2
-505
[Ru(bpy)(dpq)2](PF6)2
-415
[Ru(bpy)2(dpb)](PF6)2
-335
[Ru(bpy)(dpb)2](PF6)2
-250
bpy8
-2280
dpq8
-1430
dpb8
-1140
bpy = 2,2’-bipyridine
dpq = 2,3-bis(2-pyridyl)quinoxaline
dpb = 2,3’-bis(2-pyridyl)benzoquinoxaline
The second and third reduction potentials are impacted by the fact that a prior
reduction has occurred. Many bis-bridging ligand complexes have degenerate * orbitals, one
based on each equivalent bridging ligand. A one electron reduction will populate the * orbital
of one of the BLs, resulting in the destabilization of the other BL based * orbital. The E1/2
for [Ru(bpy)(dpq)2](PF6)2 is -415 mV, but the reduction of the second bridging ligand occurs
at -650 mV. Reduction of the first bridging ligand, hinders backbonding to that BL and results
in enhanced backbonding to the remaining bridging ligand.8 Additionally, the different
reduction potentials for equivalent polypyridyl ligands could result from electronic coupling of
the two polypyridyl ligands as facilitated by the ruthenium metal center.8
69
The trends for the second and third reduction potentials follow a similar pattern to that
seen for the first reductions with dpb reducing prior to dpq and bpy. The third reductive
couples are quasi-reversibility. This often results when the adsorption of the electrochemically
generated neutral analyte onto the electrode surface is possible.
The oxidation potential for these types of complexes reflects the electron density on
the ruthenium metal center since this oxidation represents the RuII/III couple. Ligands which
donate electron density to the metal destabilize metal based orbitals and decrease the oxidation
potential. The order of -donation of the three ligands is bpy > dpq > dpb. Ligands which
accept electron density from the metal stabilize the metal based orbitals and increase the
oxidation potential. The order of metal to ligand * backbonding is dpb > dpq > bpy. Both 
and  effects contribute to the observed oxidation potential. These effects are illustrated by the
following data: E1/2 oxidation for [Ru(dpb)3]+2 (1910 mV)9 > [Ru(bpy)(dpb)2](PF6)2 (1900
mV) > [Ru(bpy)(dpq)2](PF6)2 (1850 mV) > [Ru(bpy)2(dpb)](PF6)2 (1755 mV >
[Ru(bpy)2(dpq)](PF6)2 (1700 mV). This trend suggests that the increased degree of backbonding dominates the E1/2 oxidation potential observed.
Electronic Absorption Spectroscopy and Spectroelectrochemistry
The assignment of electronic transitions is of key importance in the characterization of
compounds which act as photosensitizers. UV-Vis absorption spectroscopy gives the energy
and extinction coefficients of these electronic transitions. Spectroelectrochemistry is a method
often used to verify these assignments in systems of this type possessing many acceptor
orbitals with well defined redox behavior. Additionally, spectroelectrochemistry probes the
70
electrochemical reversibility of the redox processes on a time scale of minutes. To date, there
has not been a complete and unambiguous characterization of the electron absorption
spectrum of any ruthenium bridging ligand polypyridyl complex.
Ruthenium polypyridyl bridging ligand complexes are expected to display a number of
electronic transitions. For [Ru (Ln)(BL3-n)]+2 complexes, the lowest energy visible transition
will a Ru (d) BL (1*) charge transfer transition. If the complex contains three of the same
BLs or three bpys, then there will be only one low energy MLCT observed. It is expected that
the molar absorptivity for this MLCT would be higher than for a bis-BL complex or bis-bpy
complex due to the increased transition probability. Higher energy MLCTs, Ru (d) BL
(2,3…*) are expected to occur, but are often difficult to locate and assign. For mixed-ligand
complexes of the formula [Ru (BL)2(bpy)]+2 and [Ru(BL)(bpy)2]+2, a second low energy
MLCT, Ru (d) bpy (1*) will be seen at energies higher than the Ru (d) BL (1*)
charge transfer transition.
Intraligand transitions (IL) are also expected for each polypyridyl ligand, they can be
more difficult to assign. It is known from comparisons of the electronic absorption
spectroscopy of similar complexes that the bpy   1* transition occurs at ca 286 nm. A BL
based   1* transition will be of lower energy than a bpy  1* transition. A BL based n
 1* transition should be found at a lower energy than the BL based   1* transition.
Higher energy intraligand manifolds also exist, such as bpy  
2*. Furthermore, transitions are not always seen as distinct peaks, shoulders from one
transition frequently are superimposed upon peaks from another transition.
71
Metal centered (or ligand field) transitions represent the excitation of an electron from
the Ru (d) to the Ru (d*) orbital. For the complexes in this study, these transitions are seen
in the UV portion of the spectrum.
The complex nature of the electronic absorption spectrum of these systems indicates
the potential utility of spectroelectrochemistry to unravel the nature and energy of the
observed transitions. As seen in the discussion above, many electronic transitions are possible
for these types of complexes.
In order to clearly probe the impact of the redox change on the electronic
spectroscopy observed, one needs to consider the nature of the optical transitions and redox
processes. Table 4.2 gives a list of possible electronic transitions as well as the expected
impact on each transition as a result of metal oxidation or ligand reduction. The first column
in Table 4.2 lists possible electronic transitions for polyazine complexes of ruthenium.
72
Table 4.2. Electronic transitions of polyazine complexes of ruthenium and the impact of metal
oxidation or ligand reduction upon these transitions.
Type of
Expected impact
Expected impact
Electronic transition
of RuII/III oxidation
of 1st BL0/-1 reduction
Ru (d) BL (1*) CT
blue shift out of visible
blue shift out of visible
Ru (d) BL (2,3,…*) CT
blue shift out of visible
blue shift
Ru (d) 2nd BL (1*) CT
blue shift out of visible
red shift
Ru (d)2nd BL (2,3,…*) CT
blue shift out of visible
red shift
Ru(d) bpy (1*) CT
blue shift out of visible
red shift
Ru(d) bpy (2,3..*) CT
blue shift
red shift
1st BL 1* IL
slight red shift
blue shift out of visible
1st BL 1,2,3,… * IL
slight red shift
blue shift
2nd BL 1* IL
slight red shift
slight red shift
2nd BL 2,3,…* IL
slight red shift
slight red shift
1st BL n1* IL
slight red shift
blue shift out of visible
1st BL n1,2,3,…* IL
slight red shift
blue shift
2nd BL n1* IL
slight red shift
slight red shift
2nd BL n2,3,…* IL
slight red shift
slight red shift
bpy  1* IL
slight red shift
slight red shift
bpy  2,3…* IL
slight red shift
slight red shift
LF or metal centered
blue shift
red shift
This table of predictions is based on others work and orbitals involved the processes.
The second column of Table 4.2 lists the spectral changes expected upon the RuII/III oxidation.
MLCTs will be strongly blue shifted, out of the visible and possibly UV due to the
stabilization of the Ru (d) orbitals upon oxidation. Gordon et al.10 propose that a new LMCT
73
appears at low energy as a result of the electrochemical generation of the Ru(III) species. The
ligand based    * transitions will be red shifted after ruthenium oxidation due to decreased
backbonding stabilizing the * orbitals.
The third column of Table 4.2 lists the expected spectral changes resulting from a one
electron reduction of the 1st BL, which results in the occupation of the BL * orbital. The
lowest energy MLCT involving this BL * acceptor orbital in the parent complex is shifted to
higher energy out of the observed window. If the complex is of a bis-BL type, the second still
unreduced BL will retain an unoccupied low energy * orbital resulting in a red shifted
MLCT. The reduction of the first BL destabilizes the remaining BL and bpy * orbitals slightly
and more drastically destabilizes the ruthenium based d orbitals. This leads to a red shift of
the other MLCT transitions based on other acceptor ligands.
The lowest energy BL (n or ) 1* transition should be blue shifted out of the visible
range upon reduction of that ligand. The remaining BL and bpy based  1* transitions can
also be impacted by the increased electron density on the metal center by BL reduction. This
could yield a minor red shift of these IL transitions. Metal centered transitions (d  d or LF)
are LaPorte forbidden giving rise to their low intensity. Their location in our BL complexes,
should be blue shifted in comparison to [Ru(bpy)3]+2 due to the decrease in -donation and
increase in -acceptance of the BLs as compared to bpy. These transitions should be impacted
more by metal oxidation than BL reduction.
74
UV-Vis Spectra of [Ru(bpy)2(dpq)](PF6)2
As mentioned in the introduction, this compound was first synthesized by Rillema and
Mack in 1982.2b Rillema’s electronic absorption assignments were based on UV-Vis data from
the Ru(II) complex. See Table 4.3.
Table 4.3. UV-Vis electronic absorption spectra of [Ru(bpy)2(dpq)](PF6)2 , in acetonitrile, at
T = 20+/- oC, according to Rillema and Mack.2b
Compound
[Ru(bpy)2(dpq)](PF6)2
max (nm)

-1
(M cm )
517
8.5x103
Ru(d)dpq(1*) MLCT
426
8.7x103
Ru(d)bpy(1*) MLCT
391(sh), 350(sh)
Ru(d) (2)* MLCT
  *
325 (sh)
284
a
Assignmenta
-1
7x104
  *
If ligand is not specified, then assignment could be BL or bpy.
The UV-Vis spectra of [Ru(bpy)2(dpq)](PF6)2 are seen in Figure 3.20-3.24, recorded in
CH3CN. The lowest energy transition at 516 nm has been assigned as Ru (d) dpq (*)
MLCT by Rillema and Mack.2b The second lowest energy peak at 426(sh) nm has been
assigned Ru (d) bpy (*) MLCT. Rillema et al.2c have postulated that the shoulders at 390
nm and 350 nm are higher energy MLCTs of the d  2* type. Additionally, there were high
intensity peaks at 200, 254 and 284 nm which were assigned intra-ligand  * transitions.
These could be based on BL or bpy.
75
Spectroelectrochemistry of [Ru(bpy)2(dpq)](PF6)2
Gordon et al.10 published the UV-Vis spectra accompanying the RuII/III oxidation and
dpq0/-1 reduction. From these results he has made the following assignments shown in Table
4.4.
Table 4.4. UV-Vis absorption spectra of [Ru(bpy)2(dpq)](PF6)2 , in acetonitrile, at T= 25 oC,
according to Gordon et al.10
Compound
[Ru(bpy)2(dpq)](PF6)2
max (nm)
515

-1
(M cm )
8.1x103
Ru(d)dpq(1*) MLCT
Ru(d)bpy(1*) MLCT
425
284
Assignmenta
-1
6.8x104
  *
This study has revealed some interesting aspects of the spectroscopy of these types of
complexes. It was not clearly indicated which electrochemical processes were reversible,
which hinders interpretation of the resultant species. There are two shoulders present in the
spectrum between 300 and 400 nm which have not been assigned. Upon oxidation of the
metal center Gordon et al. assigned the peak at 544 nm as a LMCT and observed a new
unassigned peak at 375 nm.
The following spectra were obtained in this study, the interpretations and assignments
were made in light of the current data and previously cited papers.
Impact of Oxidation at 1.90 V
In Figures 3.20 and 3.21 the peaks at 520 nm and 424 nm are lost upon the oxidation
of Ru(II) to Ru(III) at 1.90V. The shoulder at ~350 nm is lost, while a peak at 382 nm appears.
76
The fate of the shoulder at ~390 nm is unclear due to the new peak at 382 nm. The peak at 286
nm is lost and a peak at 308 nm appears. Note that a peak at ~540 nm formed with the H-cell
(Figure 3.20), but not with the OTTLE cell (Figure 3.21).
This oxidation of the ruthenium center is > 99% reversible with the OTTLE cell as seen in
Figure 3.21.
Interpretation
The loss of the 520 nm and 424 nm peak, upon ruthenium oxidation, supports the
*
*
assignments Ru (d)  dpq (1 ) and Ru (d)  bpy (1 ) MLCT transitions, respectively.
*
The red shift in the peak from 286 to 308 nm, tentatively assigned bpy  1 supports the
hypothesis that the stabilization of the ruthenium d orbitals upon oxidation decreases the
*
extent of backbonding with the ligands and decreases the  energy.
The peak at 382 nm likely represents a red shifted dpq based  1* transition. This
transition would then originate at the shoulder ca 350 nm in the parent complex. The ca 390
nm shoulder, which was lost upon oxidation, lends support to Rillema’s 2c assignment of a
higher energy MLCT. Significantly, the energy peak at 540 nm that was observed by Gordon
et al.10 and by this author (using the H-cell) is absent in the OTTLE oxidation experiment.
This suggests that the supposed LMCT at 540 nm results from incomplete oxidation.
As mentioned above, this oxidation of the ruthenium center is > 99% reversible with
the OTTLE cell as seen in Figure 3.21. The isobestic points at ~332 nm , ~370 nm and ~405
nm are indicative of a clean conversion with absence of any light absorbing degradation
products.
77
Impact of First Reduction at –1.00 V
(see Figure 3.22 for H-cell and Figure 3.23 for OTTLE cell). The reduction is 94% reversible
with the OTTLE cell as seen in Figure 3.23. Several isobestic points are present. Upon a one
electron reduction at –1.00 V to reduce the dpq ligand, the peaks at 516 nm and 424 nm lost.
Peaks at 328 nm and 483 nm are formed. The peak at 286 nm is red shifted to 292 nm. The
peak at 226 nm does not change.
Interpretation
(Gordon did not discuss the results of the dpq reduction.) As a result of a one electron
reduction at –1.00 V, the dpq * orbital is occupied. This shifts the lowest energy MLCT, Ru
*
(d)  dpq (1*) at 516 nm and the dpq   1 IL beyond the high energy window at 200
nm. The Ru (d)  bpy (1*) MLCT and bpy  * transitions should only experience minor
shifts. The absence of the 424 nm peak and appearance of the peak at 483 nm is consistent
with a red shifted Ru (d)  bpy (1*) MLCT. This author finds this shift surprising as it
would be expected that increased backbonding would destabilize the bpy * and cause a blue
shift of transitions using this acceptor orbital. The experimental result suggests that the metal
centered orbitals must be significantly destabilized by the dpq reduction and decreased
backbonding. This is further supported by the fact that this MLCT is red shifted 59 nm,
*
whereas the bpy   1 is red shifted only 6 nm (from 286 to 292 nm). The new peak at 328
nm is neither a Ru (d)  dpq (1*) MLCT nor dpq   1* IL. An assignment as a Ru (d)
78
 bpy (2*) MLCT is reasonable. This would suggest that its location prior to reduction
would have been slightly to the blue, it exact location being obscured by the large peak at 286
nm.
Impact of Second Reduction at –1.55 V
(see Figure 3.24). The second reduction was not reversible. The previously assigned
*
bpy  1 transition (286 nm) red shifts to 296 nm on the second reduction. The Ru (d)
*
bpy (1 ) at ~ 500 nm is not prominent and a subtle shoulder at ~530 nm has appeared. The
peak at 360 nm has red shifted to 370 nm.
Interpretation- (There are no examples of the second reductive spectra of this complex in the
*
literature.) The reduction at –1.55 V should result in the loss of the Ru (d)  dpq (1 )
*
MLCT and one Ru (d)  bpy (1 ) MLCT. The lowest energy transition at ~ 530 nm should
*
*
be the Ru (d) 2nd bpy (1 ). The peak at 370 nm should be the Ru (d)  dpq (2 ) or bpy
*
(2 ) MLCT. The bpy   1 transition red shifts ( to 296 nm) as expected.
79
Table 4.5. Summary for The UV-Vis electronic absorption spectra of [Ru(bpy)2(dpq)](PF6)2 in
acetonitrile at room temperature.
max

Assignment
Source
8.5x103
Ru(d)dpq(1*) MLCT
Rillema
8.7x103
Ru(d)bpy(1*) MLCT
Rillema
391 (sh)
Ru(d) (2)* MLCT
Rillema
350 (sh)
Ru(d) (2)* MLCT
Rillema
dpq   1*
Duchovnay
dpq   1*
Rillema
dpq   2*
Duchovnay
bpy   1*
Rillema
dd
Duchovnay
(nm)
(M-1 cm-1)
517
426
325 (sh)
284
7x104
250
The UV-Vis Spectrum of [Ru(bpy)(dpq)2](PF6)2
As mentioned in the introduction, this compound was first synthesized by Rillema et
al.2c in 1986. Rillema’s electronic absorption assignments were based on the UV-Vis spectra
for the parent Ru(II) complex. See Table 4.6.
80
Table 4.6. UV-Vis electronic absorption spectra of [Ru(bpy)(dpq)2](PF6)2 , in acetonitrile, at
T= 20+/-1 oC, According to Rillema et al.2c
Compound
[Ru(bpy)(dpq)2](PF6)2
max (nm)

-1
Assignmenta
-1
(M cm )
512
9.6x103
Ru(d)dpq(1*) MLCT
462
8.3x103
Ru(d)bpy(1*) MLCT
390 (sh)
Ru(d)(2*) MLCT
330 (sh)
  1*
281
6.9x104
256 (sh)
a
  1*
  2*
If ligand is not specified, then assignment could be BL or bpy.
The UV-Vis spectra of [Ru(bpy)(dpq)2](PF6)2 , synthesized by the Brewer group, is
seen in Figures 3.25 to 3.27. Rillema et al.2c made the following assignments. The Ru (d)
dpq (1*) MLCT was found at 512 nm , Ru (d) bpy (1*) MLCT at 462 nm, a higher
*
*
*
energy MLCT Ru (d)2 at 390 nm (sh) and both dpq  1 and bpy   1
transitions at 330 (sh) nm, 281 nm and 256 (sh) nm.
Spectroelectrochemistry of [Ru(bpy)(dpq)2](PF6)2
Rillema did not publish any spectroelectrochemical data on this compound. Nor did
Gordon. The following results are therefore the first presented spectroelectrochemical data on
[Ru(bpy)(dpq)2](PF6).
81
Impact of Oxidation at 1.80 V
See Figure 3.25 . The oxidation(RuII/III) was ~97% reversible. The two MLCTs at 512
nm and 462 nm are gone, but a small plateau still remains in this visible wavelength range.
The peak at 282 nm red shifts to 300 nm. There is a small shoulder peak at ~260 nm which is
lost and a small peak at ~245 nm which develops. There are also two shoulders at around 330
nm and 390 nm which are lost while a distinct peak at 368 nm appears.
Interpretation
The interpretations presented are made in light of our data, Rillema’s spectroscopic
assignments and the spectroelectrochemical properties of the above discussed
[Ru(bpy)2(dpq)](PF6). Oxidation removes one electron from the metal and transforms the
ruthenium d6 to d5. This will lower the energy of the Ru (d) orbitals and shift the Ru (d) dpq
*
*
(1 ) MLCT and Ru (d) bpy (1 ) MLCT out of the visible range. Thus it would be
*
reasonable to assign the Ru (d) dpq (1 ) MLCT to the 520 nm peak and the 466 nm peak
*
to the Ru (d) bpy (1 ) MLCT in the spectrum of the parent complex. If the shoulders at
390 nm represents a higher energy MLCT, it would be expected that they would shift out of
the visible upon metal oxidation, which is slightly discernable from the spectra . As with the
*
prior compound, the bpy   1 is red shifted (from 292 nm to 300 nm) due to reduced
backbonding of the metal center. The peak at 368 nm is the red shifted dpq   1
*
transition. The shoulder at ~260 nm is clearly gone, but whether it is blue-shifted to ~245 nm
is not certain.
82
Impact of the First Reduction at –0.70 V
See Figure 3.26. Reduction by one electron results in the occupation of the * orbital
on one of the dpq ligands. The reduction was 85% reversible. The previously assigned bpy 
*
1 at 282 nm is red shifted to 288 nm upon dpq reduction. The shoulder at ~256 nm is now
a distinct peak at 256 nm. The shoulder from 320 nm to 350 nm is transformed into a distinct
peak at 344 nm. The shoulders from 380 nm to 400 nm and the MLCT at 514 nm and 462 nm
are lost. Weak shoulders appear at 564 nm and 448 nm.
Interpretation
The first, one electron reduction (-0.70 V) should reduce one of the two dpq ligands.
This should shift the lowest energy Ru (d)dpq (1*) MLCT out of the visible region. The
remaining unreduced dpq ligand should have a 1* orbital that is destabilized due to increased
backbonding from the more electron rich metal center. As in Ru(bpy)2(dpq)](PF6)2 , the
ruthenium d should also be destabilized due to decreased backbonding. The resulting Ru
(d)  dpq (1*) MLCT will be shifted according to which destabilization is greater. The
resulting dpq MLCT should also be of reduced intensity because there is now only one LUMO
dpq acceptor orbital available. If the pattern seen in [Ru(bpy)2(dpq)](PF6)2 applies with this
analog, the resulting Ru (d)  dpq (1*) MLCT will be red shifted. The new shoulder at ca
564 nm is the logical location for this transition. The Ru (d)  bpy (1*) MLCT was at 463
nm prior to reduction. This MLCT should be red shifted. Its location is not obvious from the
spectra. The dpq  1* should be red shifted, thus explaining the new peak at 350 nm. The
appearance of a more distinct peak at 256 nm is interesting. If this is a bpy  2* transition,
83
then it implies that only the bpy 1* is destabilized and that the bpy 2* is unchanged, which is
unlikely. The peak newly revealed by dpq reduction is most likely a metal centered transition.
Impact of the Second Reduction at –1.00 V
See Figure 3.27. Reduction by a second electron should result in the occupation of the
1* orbital of the second dpq ligand. This reduction was not reversible. After reduction, there
are no distinct peaks at wavelengths greater than 360 nm. There are one or two small peaks
from 335 nm to 355 nm. The bpy  1* transition is red shifted from 288 nm to 294 nm.
The enigmatic peak at 256 nm blue shifts to 252 nm and doubled in intensity.
Interpretation
The second, one electron reduction should shift the Ru (d) (1*) dpq MLCT
transition out of the visible region due to the occupation of both dpq 1* orbitals. The dpq  
1* transition should also be shifted out of the observable range. The Ru (d) bpy (1*)
MLCT should be the lowest energy transition. Its presence is not discernable. The large
increase in intensity may indicate degradation of the analyte. The blue shift of the 256 nm
peak to 252 nm is consistent with the metal centered (LF) assignment.
84
Table 4.7. Summary of UV-Vis electronic absorption spectra of [Ru(bpy)(dpq)2](PF6)2 in
acetonitrile.
max

-1
-1
Assignmenta
Source
(nm)
(M cm )
512
9.6x103
Ru(d)dpq(*) MLCT
Rillema
462
8.3x103
Ru(d)bpy(*) MLCT
Rillema
390 (sh)
Ru(d) (2)* MLCT
Rillema
330 (sh)
(dpq)   1*
Rillema
(bpy)   1*
Rillema
  2*
Rillema
d d
Duchovnay
6.9x104
281
256 (sh)
a
If ligand not specified, it could be BL or bpy.
Comparison between [Ru(bpy)2(dpq)](PF6)2 and [Ru(bpy)(dpq)2](PF6)2:
The Ru (d) dpq (1*) MLCT occurs at 5 nm higher energy for the bis-dpq complex
vs the bis-bpy complex. The E1/2, RuII/III- dpq0/-1 is the same for both compounds. The Ru
(d) bpy (1*) MLCT is at a lower energy (462 nm) for [Ru(bpy)(dpq)2](PF6)2 than
[Ru(bpy)2(dpq)](PF6)2 (424 nm). This result is logical, there is less backbonding with the bpy
due to the bis-dpq BL. The Ru (d) (2*) MLCT occurs at the 390 nm shoulder for both
complexes. (It is not presently possible to determine which is the acceptor ligand, bpy or dpq).
According to Rillema’s assignments, the dpq  1* for bis-dpq was a 330 nm shoulder vs a
325 nm shoulder for the bis-bpy compound. In this study, the bis-bpy complex dpq  1*
was assigned to the shoulder at 350 nm. Thus the compound with the higher energy metal to
bridging ligand MLCT also has a higher energy dpq  1* transition. The same pattern is
85
seen with the bpy  1* transition, where the bis-dpq compound had a peak at 281 nm vs
284 nm for the bis-bpy analog. A comparison of the metal centered transition shows that the
bis-dpq complex has a lower energy transition (a shoulder at 256 nm) vs a shoulder at 250 nm
for the bis-bpy analog. ([Ru(bpy)3](PF6)2 also has a metal centered transition at 250 nm.) In
the light of these results no explanation is offered as to the small differences in the absorption
peaks of metal centered transitions. A comparison of the molar absorptivities of the Ru (d)
dpq (1*) MLCT is interesting. It would be expected that if these transitions are largely
localized then the bis- dpq compound should have double the molar absorptivity due to the
presence of two degenerate bridging ligand based acceptor orbitals. The data from Rillema et
3 -1 -1
al. Shows that the bis-dpq compound’s = 9.6 x 10 M cm whereas the mono-dpq
3 -1 -1
compound’s = 8.4 x 10 M cm .
The reversibilities of the oxidation and first reduction spectra of [Ru(bpy)2(dpq)](PF6)2
were very high, 100% and 94% respectively. In contrast, for [Ru(bpy)(dpq)2](PF6)2, the
oxidation spectra was ~97% reversible but the first reduction spectra was only 85% reversible.
The reproducibility of reversibility was inconsistent and varied with individual attempts. The
lack of reversibility for the second and third reductions was consistent.
UV-Vis Spectrum of [Ru(bpy)2(dpb])(PF6)2
(see Figure 3.19). As mentioned in the introduction, this compound was first
synthesized by Murphy et al.9 in 1991. They assigned the lowest energy peak at 550 nm to Ru
(d)dpb (1*) MLCT. Gordon et al.10 (1997) looked at the spectroelectrochemistry of the
RuII/III oxidation and the one electron reduction of this complex. Their assignments are shown
in Table 4.8.
86
Table 4.8. UV-Vis electronic absorption spectra of [Ru(bpy)2(dpb)](PF6)2 in acetonitrile at
25oC by Gordon et al.10
Compound
[Ru(bpy)2(dpb)](PF6)2
max (nm)
550

(M-1 cm-1)
8.3x103
Assignment
Ru(d)dpq(1*) MLCT
425 (sh)
Ru(d)bpy(1*) MLCT
392 (sh)
dpb   1*
314
4.5x104
285
6.8x104
bpy   1*
242
4.5x104
  2*
(BL or bpy)
A low energy shoulder at 425 (sh) nm was assigned as a Ru (d)bpy (1*) MLCT.
The shoulder at ~392 nm was assigned as a dpb  1* intra-ligand transition. The bpy 
1* transition occurs at 285 nm. A higher energy intra-ligand  * transition was found at
242 nm.
(The spectroelectrochemistry of [Ru(bpy)2(dpb)](PF6)2 was not performed with the OTTLE
cell due to time constraints).
Gordon et al.10 studied the spectroelectrochemistry of the oxidation and
first reduction of [Ru(bpy)2(dpb)](PF6)2 in 1997. Upon oxidation at 1.5 V, the Ru () dpb
(*) MLCT at 550 nm and Ru () bpy (1*) MLCT at 425 nm (sh) were bleached and a new
peak appeared at ~ 575 nm; it was assigned as a LMCT. Upon reduction at –0.8 V, the Ru ()
dpb (1*) MLCT was lost and the Ru () bpy (1*) MLCT was red shifted to ~ 475 nm.
87
UV-Vis Spectra of [Ru(bpy)(dpb)2](PF6)2
This new compound was determined by OSWV to be ca 99% pure. See Figure 3.12.
The Ru(II) spectra in Figures 3.28, 3.29 and 3.30 show the peak at 552 nm, but only in Figure
3.30 can the peak at 500 nm be clearly resolved. All figures show shoulders from 380-420 nm
and a smaller shoulder at ca 450 nm. The next transition is a strong peak at 316 nm. There is a
shoulder at ~ 280 nm and a noticeable peak at 242 nm.
Spectroelectrochemistry of [Ru(bpy)(dpb)2](PF6)2:
Impact of Oxidation at 1.75 V
(see Figures 3.28). The oxidation at this potential generates the Ru(III) species. The reversal of
the oxidation was incomplete, ca 10%. After oxidation, the peaks at 552 nm, 500 nm, and
shoulder from 394 nm to 415 nm, were absent.
There may be a shoulder at ~360 nm which remains after oxidation. The most
prominent peak, at 316 nm, is reduced in intensity by ~ 50% upon oxidation. There is a
shoulder at ~ 280 nm which appears to blue shift slightly. A substantial peak at 242 nm red
shifts to 248 nm and is reduced in intensity by 33%. The reversal of the oxidation was
incomplete.
Interpretation
The Ru (d)dpb (1*) MLCT should shift out of the visible region upon oxidation.
Indeed, the lowest energy peak at 552 nm is not seen after oxidation. Likewise the Ru
(d)bpy (1*) MLCT should also shift out of the visible region, this transition is assigned to
the 500 nm peak which is no longer visible. The shoulder from 394 nm to 415 nm is not seen
upon oxidation, this region should contain higher energy MLCTs, which also should shift out
88
*
of the visible after oxidation of the metal center. The dpb  1* and bpy  1 should be
*
assignable, with the dpb transition being of lower energy. The bpy   1 transition has in
prior compounds been seen at ~ 286 nm. This peak is seen here as a shoulder, being obscured
by the large absorption at 316 nm.
Impact of the First Reduction at –0.50 V
(see Figure 3.29). Reduction by one electron leads to the reduction of one of the two
dpb ligands. The reduction was > 90% reversible. The lowest energy transitions at 554 nm and
500 nm are lost and a new peak at 590 nm appears. The significant shoulder at ~390 nm may
be blue shifted to ~378 nm. The intense peak at 316 nm has red shifted to 318 nm and
decreased in intensity. Prior to reduction, a small shoulder at 280 nm was superimposed upon
the 316 nm peak. This small peak became more clearly defined upon the diminishment of the
dominant peak. The noticeable peak at 240 nm did not change.
Interpretation
The first, one electron reduction should reduce one of the dpb ligands. This should
*
shift both the Ru (d)dpb (1 ) MLCT and dpb   1* intra-ligand transition out of the
visible region. A destabilization of the dpb based orbital on the second ligand is also expected.
Destabilization of the Ru (d) orbitals upon ligand reduction red shifts the remaining Ru
*
*
(d)dpb (1 ) MLCT to 590 nm. The Ru (d)bpy (1 ) MLCT should also red shift
from 500 nm, but its location is not obvious. This transition and the remaining Ru (d)dpb (
1*) MLCT are probably overlapping. A shoulder appears at ca 380 nm and is of unknown
*
origin. The shoulder at ~ 286 nm, which is the bpy  1 transition, remains relatively
89
*
unaffected. The peak at 316 nm is not likely to be dpb   1 because the dpq   1* in
the bis-dpq analog is seen at 330 nm. It could be a higher energy IL transition, dpb  2*.
Impact of the Second Reduction at –1.0 V
(see Figure 3.30). The second reduction led to a species in which both dpb ligands are
reduced by one electron. This reduction was not reversible. Generation of the two electron
reduced species gives a low intensity shoulder at 360 nm. This is about an 18 nm blue shift
from the first reduction peak location. The prominent peak at 318 nm is gone. The peak at
~286 nm remains. A new peak at ~ 258 nm appears. The peak at 240 nm is gone.
Interpretation
After two one electron reductions, the only Ru (d) (1*) MLCT likely is the Ru
(d)bpy (1*) MLCT. This transitions location is obscured in a region of multiple
*
absorbance peaks. The (lowest energy) dpb  1 transition is shifted out of the observable
*
range. The second dpb   1 transition is similarly shifted. This data suggests that the 318
nm peak in the parent species is the dpb   2* transition. It is not clear if the 240 nm peak
has red shifted to ~258 nm or whether the former is lost and the latter represents a different
transition. In lieu of the similarity between this complex and the three previously discussed
complexes, the assignment of d d for the transition at 240 nm in the parent species would be
reasonable.
90
Table 4.9. Summary of UV-Vis electronic absorption spectra of [Ru(bpy)(dpb)2](PF6)2, in
acetonitrile, at room temperature.
max (nm)
Assignment (Duchovnay)
552
Ru (d)dpb (1*) MLCT
500
Ru (d)bpy (1*) MLCT
ca 425 (sh)
Ru (d)bpy or dpb (2*) MLCT
ca 390 (sh)
dpb   1*
316
dpb   2*
285
bpy   1*
242
dd
Comparison between [Ru(bpy)2(dpb)](PF6)2 and [Ru(bpy)(dpb)2](PF6)2
*
The Ru (d) dpb (1 ) MLCT occurs at the same wavelength, 550 nm, for both
complexes. The Ru (d) bpy (1*) MLCT peak is seen at 500 nm for the bis- dpb complex
and 425 nm (sh) in the bis-bpy complex. This pattern was also seen in the two dpq
*
analogs.The dpb  2 IL transition was assigned to the peak at 316 nm for the bis-dpb
complex by this author. The location of the dpb   1* was obscured by overlapping
absorbance peaks in the 390 nm region. Gordon et al.10 assigned the 392 nm (sh) to the dpb 
*
 1* for the bis-bpy complex. The bpy   1 transition (~286 nm) seems to be consistent
for both complexes. Both complexes have peaks at 242 nm, which has been designated as d 
*
d by this author as compared to  1 IL by Gordon et al.10 In light of these comparisons the
following summary of the electronic transitions for the bis-bpy complex is shown in Table
4.10.
91
Table 4.10. Summary of UV-Vis electronic absorption spectra of [Ru(bpy)2(dpb)](PF6)2 in
acetonitrile.
max (nm)

Assignment
Source
Ru(d)dpq(1*) MLCT
Gordon
425 (sh)
Ru(d)bpy(1*) MLCT
Gordon
392 (sh)
dpb   1*
Gordon
????
Gordon
dpb   2*
Duchovnay
bpy   1*
Gordon
(M-1 cm-1)
8.3x103
550
4.5x104
314
4.5x104
(Gordon)
285
6.8x104
242
4
4.5x10
  2*
Gordon
(BL or bpy)
4.5x104
d d
Duchovnay
Comparison between [Ru(bpy)(dpq)2](PF6)2 and Ru(bpy)(dpb)2](PF6)2
The Ru (d) dpb * MLCT is lower in energy for the dpb complex, 552 nm vs 512
nm for the dpq complex. This is an expected result as the dpb * acceptor orbital is lower in
energy compared to the dpq counterpart. The Ru (d) bpy * MLCT is lower in energy for
the dpb complex, 500 nm vs 462 nm for the dpq complex. This result is consistent with
decreased backbonding between the metal center and the bpy * acceptor orbital in the bis-dpb
complex. The same pattern follows for higher energy MLCTs, 420 nm for the dpb complex vs
390 nm for the dpq complex. All IL transitions are lower in energy for the dpb complex, as is
the d  d transition.
92
References
1.
Denti, G.; Campagna, S.; Sabatino, L.; Scolastica, S.; Ciano, M.; Balzani, V.Inorg.
Chem. 1990, 29, 4750.
2.
(a) Rillema, D. P.; Callahan, R. W.; Mack, K. B. Inorg. Chem. 1982, 21, 2389. (b)
Rillema, D. P. and Mack, K. B. Inorg. Chem. 1982, 21, 3849. © Rillema, D. P.;
Taghdiri, D. G.; Jones, D. S.; Keller, C. D.; Worl, L. A.; Meyer, T. J.; Levy, H. A. Inorg.
Chem. 1987, 26, 578.
3.
(a) Bridgewater, J. S.; Vogler, L. ; Molnar, S.; Brewer, K. Inorg. Chim Acta. 1993, 208,
179. (b) Richter, M. M.; Brewer, K. J. Inorg.Chem. 1993, 32, 5762.
4.
Wang, J. Analytical Electrochemistry, 1994, 53, VCH Publishers, New York.
5.
(a) Bridgewater, J. S.;Vogler, L. M.; Molnar, S.M.; Brewer, K. J. Inorg. Chim. Acta.
1993, 208, 179. (b) Richter, M. M.; Brewer, K. J. Inorg. Chem. 1993, 32, 5762.
6.
Bard, A.; Faulkner, L. Electrochemical Methods. 1980, 229, John Wiley, New York.
7.
Vogler, L. M., Ph.D. Dissertation. YEAR? 63.
8.
Donohue, R. J.; Tait, C. D.; DeArmond, M. K.; Wertz, D. W. Spectrchim. Acta.
1986, 42, 233.
9.
Carlson, L. C. and Murphy, W. R. Jr. Inorg. Chim. Acta. 1991, 181, 61.
10. Scott, S.; Gordon, K. Inorg. Chim. Acta. 1997, 254, 267.
93
CHAPTER FIVE: CONCLUSIONS AND FUTURE WORK
This work is composed of synthetic, electrochemical and spectroelectrochemical
studies. A new compound, [Ru(bpy)(dpb)2](PF6)2 has been prepared and its purity and identity
were analyzed with OSWV and LSIMS. Purification was difficult, with a ca 1% impurity
remaining after repeated alumina adsorption chromatographs. The spectroscopic and redox
properties of the new complex and three related dpb and dpq systems were probed with cyclic
voltammetry, electronic absorption spectroscopy, and spectroelectrochemistry. The
spectroelectrochemical studies were conducted with both a bulk H-cell and OTTLE cell. The
significant conclusions of this study are highlighted.
Synthesis
(1) The new compound [Ru(bpy)(dpb)2](PF6)2 was synthesized.
(2) LSIMS verified its identity.
(3) OSWV indicated presence of ca 1% electroactive impurity after repeated alumina
adsorption chromatographs.
(4) The previously studied compounds [Ru(bpy)2(dpb)](PF6)2, [Ru(bpy)2(dpq)](PF6)2 , and
[Ru(bpy)(dpq)2](PF6)2 were prepared for more detailed analysis.
(5) An attempt was made to probe the purity and identity of the starting material RuCl33H20
and intermediate Ru(bpy)Cl4. The LSIMS of the starting material RuCl33H20 (see Appendix
Figure 3.8) did not reveal the parent ion. The peak at m/z=137 is evidence of RuCl+. The
LSIMS of Ru(bpy)Cl4 (see Appendix Figure 3.6) did not contain the parent ion, m/z = 400,
and thus did not confirm the composition. Upon washing Ru(bpy)Cl4 with ethanol, two
fractions were produced, a soluble brown filtrate and a relatively insoluble black solid. The
94
UV-Vis spectra of these two fractions , as seen in Appendix Figure 3.9, are significantly
different. Anderson and Seddon1 determined by elemental analysis that Krause’s2 synthesis,
used herein, produces Ru(bpy)Cl3 (thus implying polymeric form), whereas the protocol of
Dwyer et al.3 yielded Ru(bpy)Cl4. Although the identity of this intermediate was not
determined in this study, its utility as a precursor in the synthesis of [Ru(bpy)(dpq)2](PF6)2 and
many other mixed ligand complexes has been demonstrated. Future work should explore
alternative chromatographic methods, such as HPLC and cation exchange chromatography,
which may yield a more pure product.
Electrochemistry
(1) OSWV was used to ascertain purity by detection of electroactive impurities, with a
detection limit of approximately 0.3%.
(2) Cyclic voltammetry was used to probe electrochemical reversibility, and E1/2 values.
(3) Contamination by water and oxygen leads to poor electrochemical reversibility, so an
anaerobic electrochemical cell with internal desiccant was constructed. Further testing of this
new cell will be part of my proposed future research.
(4) One important piece of information gained by electrochemical studies is the order of
ligand reductions for mixed ligand complexes like [Ru(bpy)(dpb)2](PF6)2. Typically each dpb
ligand will reduce by one electron followed by a one electron reduction of any bpy ligands.
Some aspects of the electrochemistry of [Ru(bpy)(dpb)2](PF6)2 seem to suggest the dpb-/2couple may occur prior to the bpy reduction. The lack of reversibility in the
spectroelectrochemistry did not allow the assignment of this third reduction. Low temperature
studies may be used in the future to address this question.
95
Spectroelectrochemical Experimental Design
(1) Spectroelectrochemical analysis requires good electronic spectroscopy as well as
high electrochemical conversion and reversibility. Several studies were undertaken to achieve
these criteria. To enhance reversibility, an OTTLE cell ( a modification of the Hartl cell4) was
constructed and used. A typical H-cell was also used for comparison. (a) Electrochemical
reversibility is significantly different between the two methods, the OTTLE cell facilitating
better reversibility, as high as 99% as compared to 50% with the H-cell. (b) The time needed
for analysis is shorter with the OTTLE cell, 1 minute for the OTTLE vs 15 minutes for the Hcell.
(2) The impact of analyte concentration on spectroelectrochemical studies was
explored. It was found that (a) the resolution of shoul ders is enhanced with increasing
concentration and (b) the time of analysis and degree of irreversibility increase with increased
analyte concentration. The protocol for (a) requires high concentrations whereas the protocol
for (b) necessitates low concentrations.
Summary of Spectroelectrochemistry Conclusions
(1) The electronic transitions of the new complex [Ru(bpy)(dpb)2](PF6)2 have been
presented in Table 4.9. The spectroelectrochemistry of the metal oxidation, first and second
one electron reduction of the BL has been presented. The oxidation was only 10% reversible
whereas the 1st reduction was 90% reversible. The poor reversibility of the oxidation was
unexpected and warrants further research. Although the 2nd reduction was not reversible,
spectral results provided valuable information supporting assignment of the peak at 316 nm as
dpb   2*.
96
(2) The electronic transitions, supported by spectroelectrochemical analysis, of the
related bis-BL complex [Ru(bpy)(dpq)2](PF6)2 have been presented in Table 4.10. The
spectroelectrochemical oxidation of the metal center was 97% reversible. The 1st one electron
reduction of the BL was 85% reversible. The 2nd one electron reduction of the 2nd BL was not
reversible.
(3) When complete oxidation of the metal center of [Ru(bpy)2(dpb)](PF6)2,
[Ru(bpy)2(dpq)](PF6)2 , [Ru(bpy)(dpb)2](PF6)2 and [Ru(bpy)(dpq)2](PF6)2 is achieved, using
the OTTLE cell, no low energy transitions are left. This indicates that this transition which
Gordon et al.6 assigned as a LMCT is likely a residual. This highlights the importance of
careful and complete electrochemical conversions in spectroelectrochemical studies. As
LMCTs are reasonable further studies aimed at locating them (possibly in the NIR ) are
warranted.
(4) This study has assigned the peak at ca 240-260 nm to a d  d metal- centered
transition. Rillema and Gordon assign these peaks to IL transitions.
(5) A comparison of the transitions of the two bis-BL complexes shows that the
electronic transitions of the bis-dpb complex are of lower energy than the corresponding
transitions of the bis-dpq complex.
(6) The Ru (d)  dpb (1*) MLCT for both dpb BL complexes occurred at the same
wavelength, 550 nm. In contrast, the corresponding MLCT for the bis-dpq
BL complex was seen at 512 nm as opposed to 517 nm for the mono-dpq complex.
Thus no specific conclusion can be made concerning the effect one or two BLs on the MLCT.
97
This is surprising in light of the fact that the corresponding MLCTs for the tris-BL complexes
are higher in energy than their corresponding bis-BL counterparts.
(7) The Ru (d)  bpy (1*) MLCT is of lower energy for the bis-BL complexes vs the
corresponding mono-BL complexes.
(8) It has been assumed that second lowest energy transition of the parent complex is
Ru (d)  bpy (1*) MLCT and that the third lowest energy peak is a higher energy Ru (d)
 BL (2*) MLCT. It is possible that the Ru (d)  bpy (1*) MLCT is in reality the 3rd peak
and the Ru (d)  BL (2*) is the 2nd peak. This dilemma can not be resolved with the
presently available spectroelectrochemical data. The clarification of this question warrants
further research.
(9) The bpy  * IL transition is least affected by the number and identity of the BL
when comparing the four analogs.
Future Work
Synthesis: The complex [Ru(bpy)(dpq)(dpb)](PF6)2 would a challenging compound to
synthesize.
Purification: Alternative methods of purification should be explored, such as HPLC and
cation exchange chromatography.
Electrochemistry: Electrochemical data, using CV and OSWV, need to be obtained with the
new anaerobic cell. Low temperature studies, using the anaerobic cell, also need to be
conducted.
Spectroelectrochemistry: (1) The spectroelectrochemistry of [Ru(bpy)2(dpb)(PF6)2 needs to be
done. (2) Location of shoulders (weak absorbance peaks) needs to be re-evaluated by using
98
high concentrations of analyte. With this additional data, spectroelectrochemical studies of the
four complexes needs to repeated. (3) The low reversibility of the electrochemical oxidation of
the new complex [Ru(bpy)(dpb)2](PF6)2 needs to be re-evaluated. (4) The reason for the lack
of reversibility of the 2nd and 3rd electrochemical reductions needs to be explored. (5) The
assumption that the 2nd lowest energy peak is a Ru (d) bpy (1*) MLCT warrants future
research. (6) The spectroelectrochemical analysis of the tris-BL ruthenium complexes will
yield valuable additional information. (7) A low temperature OTTLE cell needs to be built. (8)
A NIR OTTLE spectroelectrochemical cell needs to be designed and built for the analysis of
LMCTs, interligand transitions and intraligand transitions of reduced ligand complexes. It is
the hope of this author that the reader has gained some appreciation of the challenges faced in
obtaining and interpreting spectroelectrochemical data. The opportunity for future research is
abundant.
99
References
1.
Seddon, K.R.; Anderson, S. J. Chem. Res. (Synopsis). 1979, 2, 74.
2.
Krause, R. A. Inorg. Chim. Acta. 1977, 22, 209.
3.
Dwyer, F. P.; Goodwin, H. A.; Gyarfas, E. C. Aust. J. Chem. 1962,
4.
Krejcik, M.; Danek, M.; Hartl, J. J. Electroanal. Chem. 1991, 317, 179.
5.
(a) Rillema, D. P.; Callahan, R. W.; Mack, K. B. Inorg. Chem. 1982, 21, 2389. (b)
Rillema, D. P.; Callahan, R. W.; Mack, K. B. Inorg. Chem. 1982, 21, 3849. (c ) Rillema,
D. P.; Taghdiri, D. G.; Jones, D. S.; Keller, C. D.; Worl, L. A.; Meyer, T. J.; Levy, H. A.
Inorg. Chem. 1987, 26, 578.
6.
Scott, S.; Gordon, K. Inorg. Chim. Acta. 1997, 254, 267.
7.
Richter, M. M. Ph.D. Dissertation. 1993, 147.
100
Appendix Figure 1.1. Diasteriomers of [Ru(bpy)(dpb)2]+2. Each diasteriomer has an
enantiomer.
A
A
C
C
Ru
B
B
A
A
B
B
R
u
C
C
A
A
B
C
Ru
B
C
A= N of terminal ligand 2,2' bipyridine
B= N of pyridine of bridging ligand
C= N of pyrazine portion of bridging ligand
101
Appendix Figure 3.1. LSIMS+ of the matrix glycerol. MW 92+1.
102
Appendix Figure 3.2. LSIMS+ of the matrix nitro-benzyl alcohol. MW 153+1
103
Appendix Figure 3.3. LSIMS+ of free ligand bpy in glycerol. MW 156+1.
104
Appendix Figure 3.4. LSIMS+ of free ligand dpq in glycerol. MW 284+1.
105
Appendix Figure 3.5. LSIMS+ of free ligand dpb in glycerol. MW 334+1.
106
Appendix Figure 3.6. Electrospray (+) MS of Ru(bpy)Cl4 in acetonitrile/water. MW= 399.
Appendix Figure 3.5. LSIMS+ of free ligand dpb in nba. MW =334+1.
107
Appendix Figure 3.7.. UV-Vis spectroelectrochemistry of ferrocene. ( __= 0 V, ….. = 1.7 V).
108
Appendix Figure 3.8. LSIMS- of RuCl33H2O in NBA. MW= 255+1.
109
Appendix Figure 3.9. The electronic absorption spectra of the two fractions of Ru(bpy)Cl4 in
water. ( Where bpy= 2,2’ bipyridine).
110